Simplify.

a) [[$ 5x + 8y + 7x + 4y = $]]
b) [[$ 2a^2 - 9a - 4a +6 = $]]
c) [[$ 5x - 4xy + 7y + 3x + 2xy= $]]
d) [[$ 8ab - 5ab + 6ba - 4ab= $]]