Perpendicular lines have an opposite reciprocal slope, therefore, the slope of the perpendicular line is:
-1/7
So far, y = -1/7x + b. Now we need to plug in (7, 7) into x and y for our equation.
7 = -1/7(7) + b
Simplify the right side
7 = -1 + b
Add 1 to both sides
8 = b
The equation of the perpendicular line is:
y = -1/7x + 8