Show that Line (7,0),(5,8)and line (2,y),(10,5) is perpenpedicular.

m1 = 8-0 / 5-7

     = 8/-2

   m1 = -4

m2 = 5 - y /10 -2 

in order to be perpendicular m2 must be the negative reciprocal of m1..

m2 = 1/4

1/4 = 5-y/8

2 = 5 - y

y = 5 - 2 = 3