You are correct. The answer is choice C.
Step-by-step explanation:
We can solve for 'a' like so
In the second step, I multiplied both sides by 2. Afterward, I divided both sides by h in the third step. Lastly, I subtracted b from both sides to isolate 'a' in step 4. Each step is following PEMDAS in reverse to undo every operation done to variable 'a'.