slope of line ax + by = c is -a/b, so slope of line 4x - 3y = 18 is 4/3.
slope of perpendicular line is negative reciprocal or -3/4
point intercept equation of line is y-y1 = m(x - x1) where m is slope and (x1,y1) is point line passes through, so given point line passes through is (8,-8):
y +8 = -3/4(x-8)
y+ 8 = -3/4 x + 6
4y + 32 = -3x + 24 (multiply through by 4)
3x + 4y = -8 (modify to ax + by = c format)