It's a question from my End of (Maths) Module exam. I submitted it last month and I don't get feedback. I was proud of that and I'm glad I was right to be proud of it...
They're not symbol-for-symbol the same, but yes, they're equal, since 3-c is -(c-3), and 12c-6a is -(6a-12c). I had checked to make sure. You can also have Wolfram Alpha check for symbolic equality, see here.
The time you enjoy wasting is not wasted time. Bertrand Russell