To solve this you simply substitute or plug in three wherever there is an a and then isolate b.
4(3) = 2b-7 ------Simplify by Multiplying
12 = 2b - 7 ------ Add 7 to both sides
19 = 2b -------- Divide both sides by 2
19/2 = b
Your final answer is then C 19/2
Hope this helps!
