Answer:
second option, (u - 12 - 3b)/4
Step-by-step explanation:
isolate the "a" by bring each term to the other side, and finally div by 4
u = 12 + 4a + 3b
u - 12 = 4a + 3b
u - 12 - 3b = 4a
(u - 12 - 3b)/4 = a
