Answer:
second option, (u - 12 - 3b)/4
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
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