To make x the subject means to solve the equation for x (or basically to get x to stand by itself.)
First, we expand the bracket :
h = 4x + 12y + 2
Minus both sides by 12y + 2 :
h - 12y - 2 = 4x
Divide both side by 4 :
(1/4)h - 3y - 1/2 = x
Therefore, x = (1/4)h - 3y - 1/2.