The line is perpendicular, so mx is the reciprocal of 2/5 which would be -5/2. Then you plug that and (4, -3) into the equation y = mx + b, which will come out as -3 = (-5/2)(4) + b. Multiply mx to get -3 = -10 + b. Add 10 to both sides to get 7 = b. Plug in mx and b into the equation and you have your answer.
