Answer:
AB = 9
Explanation:
Since triangle ACE and BCD are congruent, they have the same angles, you can figure out the fraction to get from ACE to BCD, then reverse it. So since CE is 12 + 4 = 16 and CD is 4, we can see that ACE is 4 times greater than BCD because 16/4 = 4.
So to get AB, find AC by multiplying BC by 4, which gives 12, then subtract AC by BC to get AB: 12 - 3 = 9