Subtract:
4x^3 − 6x^2+8x − 9 from 7x^3 − 16x + 18

Also:
From the sum of a^2 − 2ab + b^2 and 2a^2 + 2ab + b^2 subtract the sum of a^2 − b^2 and a^2 + ab + 3b^2