First divide each side by 2 isolate y. Now you have y = (-9/2)x - 12. The slope is (-9/2). For a perpendicular line, flip the slope and negate it to get 2/9. Then use y = mx + b.
5 = (2/9)(4) + b
5 = (8/9) + b
(37/9) = b
So the equation is y = (2/9)x + 37/9 or
9y = 2x + 37.