When two lines are perpendicular to one another, their slopes multiplied together is always going to be -1.
With this in mind, set up the following equation:
m₁ * m₂ = -1

So

is your slope.
Since y = (-1)x + b, plug in y and x to solve for b, using the point (10, -8);

So your equation is
y = -x + 2