VBA formula Property help!

Thanks in Advance! Okay guys, I have spend a whole day trying to figure this thing out but no luck I think I'm done with part 1 and 2, but I have no clue what to do with part 3. I know I'll have to use activecell.formula, but I'm not getting anywhere. I'm attaching the file as well. The questions that I'm trying to solve for my problems are as follows:

1. The excel has sales totals for 12 months and 10 different products in the range B4:M14, write a VBA sub to enter formulas for the total in column N and row 14. Use R1C1 property to write formula.

2. Repeat previous exercise, but now assume the data set could change, either by adding more months or adding products, or both. Using Formula R1C1, fill the row below data and the column to the right of the data with formulas for totals. (Hint, use string concatenation)

3. Do the previous two exercises by using "formula" property, rather than formulaR1C1 property.

Option Explicit

Sub TotalProductSales()
    Dim numMonths As Long
    Dim numProducts As Long
    
    With Range("b4")
        numMonths = _
            Range(.Offset(0, 0), .End(xlToRight)).Columns.Count
        numProducts = _
            Range(.Offset(0, 0), .End(xlDown)).Rows.Count
            
        Range(.Offset(0, numMonths), _
                .Offset(numProducts - 1, numMonths)).FormulaR1C1 = _
            "=sum(rc[-" & numMonths & "]:rc[-1])"
    End With
    
End Sub


Option Explicit

Sub TotalMonthlySales()
    Dim numMonths As Long
    Dim numProducts As Long
    
 
    With Range("b4")
        
        If .End(xlDown).HasFormula = False Then
         numMonths = Range(.Offset(0, 0), _
                    .End(xlToRight)).Columns.Count
        numProducts = Range(.Offset(0, 0), _
                    .End(xlDown)).Rows.Count
        Range(.Offset(numProducts, 0), _
                .Offset(numProducts, numMonths - 1)).FormulaR1C1 = _
            "=sum(r[-" & numProducts & "]c:r[-1]c)"
        Else
         numMonths = Range(.Offset(0, 0), _
                    .End(xlToRight)).Columns.Count
        numProducts = Range(.Offset(0, 0), _
                    .End(xlDown)).Rows.Count - 1
        Range(.Offset(numProducts, 0), _
                .Offset(numProducts, numMonths - 1)).FormulaR1C1 = _
            "=sum(r[-" & numProducts & "]c:r[-1]c)"
        End If
        
       
    End With
    
    With Range("b4")

Select Case .End(xlDown).HasFormula
Case False
 numMonths = Range(.Offset(0, 0), _
                    .End(xlToRight)).Columns.Count
        numProducts = Range(.Offset(0, 0), _
                    .End(xlDown)).Rows.Count
        Range(.Offset(numProducts, 0), _
                .Offset(numProducts, numMonths - 1)).FormulaR1C1 = _
            "=sum(r[-" & numProducts & "]c:r[-1]c)"
        Case True
       numMonths = Range(.Offset(0, 0), _
                    .End(xlToRight)).Columns.Count
        numProducts = Range(.Offset(0, 0), _
                    .End(xlDown)).Rows.Count - 1
        Range(.Offset(numProducts, 0), _
                .Offset(numProducts, numMonths - 1)).FormulaR1C1 = _
            "=sum(r[-" & numProducts & "]c:r[-1]c)"
            End Select
            
        End With