a) = 12a^2 - 21a + 3a^2 - 1 = 15a^2 - 21a - 1
b) = -x + 5xy + 2x +xy = x + 6xy
c) = 3 - 5c + 1/2b
d) -x2 + 5x + 2x - 6 = -x^2 + 7x - 6
