If you have a triangle with the following points
A (-6,-4)
B (-1,8)
C (13,-2)

Find the point perpendicular to AC.
y-y1=m(x-x1)

For this equation, it is solved
y+4=2/19(x+6)
y=2/19x-64/19

y-8=-19/2(x+1)
y=-19/2x-3/2
Therefo
2/19x-64/19=-19/2x-3/2
365x=71
x=.195

..195 is substituted back into the orginal formula to find for y.

I know to use the x and y variable to have the function call out a cell with
either the x or y in that cell, what I believe is screwing up the function is
the = sign in the middle of it. Any suggestions?