Simplify and calculate the value of the polynomial when [[$ x = -2$]].

a) [[$2x^3 - 3x^2 + 4x + (-2x^3 +4x^2 -9x)$]]


b) [[$7x^3 + 8x - (6x^3 - 4x^2 - x) - 4x^2$]]