Since it's perpendicular to the line with the slope 5, you take the reciprocal of 5 which is 1/5 and change the sign: -1/5. So you have the equation: y=-(1/5)+b. To find b you substitute x and y for the coordinate (10,8).
y=(-1/5)x+b
8=(-1/5)10+b
8=(-10/5)+b
8=-2+b
+2 +2
10=b
So your new equation is: y=1x+10
5
