how to return a filename from a cell to msgbox

i have the following code:

    Range("T13").Select
    ActiveCell.FormulaR1C1 = _
        "=SUBSTITUTE(SUBSTITUTE(LEFT(CELL(""filename"",R[-12]C[-19]),FIND(""]"",CELL(""filename"",R[-12]C[-19]))),""["",""""),""]"","""")"
    x = =T13
    MsgBox x

but the msgbox just returns a blank dialog box, how can i get the current filename to display and use it in a vba statement?