You can use the distributive property, so that would be (Ab)-(Ac)=a/b.
After solving the little bit on the left of the 'equal sign' you need to get rid of the division on the right, so you take the 'b' to the other side and instead of dividing, you multiply it to the equation on the left. And you keep on solving util you're left with c = (something)
Hope that helped :) !