Conditional Slope

Hi,

I have data of numbers in columns A and B from A21 to A30, and B2 to B30,

I'm looking for a function that can help me to get slopes out of A and B data (data on column A as X axis, and Y on the B), the idea is that if I type a letter (like "O" ) anywhaere in the column C like in cells C2,C3, C5, C7, for example then in the cell (D2) I get the slope of the points: (A2,B2) , (A3,B3) , (A5,B5) and (A7,B7)... and if I type at the same time "W" anywhaere in different cells
on the column C I get the slope of those points in the cell D4...
I attched an example to clarify the idea...

Appreciate help!