For perpendicular lines, the product of their slopes is -1.
Let the slope of the second line be m, then 5/4 m = -1
m = -4/5
The slope of a line passing through two given points is given by m = (y2 - y1)/(x2 - x1)
-4/5 = (y - (-7))/(3 - 8) = (y + 7)/-5
4 = y + 7
y = 4 - 7 = -3
y = -3