GCSE Vectors. Urgent help needed!

If you've found the vector of AM, then for QM just halve that vector, since Q is half way along AM.

You also know MB, now just add them together.

Alright, so that would be 1/2(3a+b)+b, am I correct? But it's still not in terms of just c? Sorry to be a pain!

Oh wait sorry about that I should have thought about this more carefully. My fault

First of all QM is -1/2*(b) + 3/2(a) + b, since you have to go from Q, to the mid-point of AC, then down to C, then to M to make QM.

That simplifies to $\frac {3a}{2} + \frac{b}{2}$

If you look carefully you can also see that c = 1/2(a) + 1/2(b).

Now back to MB, that's just b, as M is the mid-point of CB. Add that to QM.

You should have a value that is easy to get by manipulating c in some way.