Algebra problem: Can you help?

Afternoon, I’m just doing a bit of algebra revision on the BBC GCSE Bitesize website and one question has stumped me: I can’t figure out where I’m going wrong.

Here is the question:

4. If a = b – c, then:

a. c = b – a
b. c = a + b
c. c = a – b

Apparently a is the right answer, but I can’t work out how to get there.

I’ve been taught that when letters/numbers change sides they change their sign as well, therefore I answered b.

Any help is appeciated :) (I could just ask my Maths teacher on Monday but I’m rather impatient)