Rearranging is one of the trickier topics in GCSE maths, however as long as you follow it through step by step you shouldn't have a problem.
The question asks you to make 'c' the subject. This means you need to get 'c' on one side of the equation, and all the other terms on the other side of the equation.
Firstly, you should multiplty everything through by 2 to get rid of the fraction:
3c + b = 2(c + a)
3c + b = 2c + 2a
Then, by adding and subtracting terms, move all of the 'c' terms on to one side of the equation and all of the other terms on to the other side of the equation:
3c - 2c = 2a - b
Therefore, your final solution is:
c = 2a - b