I'm not seeing anything super sneaky so just expand everything and collect it all back in terms of x and y.
Then solve it the way you've been doing.
3(x - a) - 2(y + a) = 5 - 4a ---> 3x - 2y = 5+a
You're final answer is going to include a in it. It's just a constant you don't happen to know the value of.