Tìm x: a) 2(x + 2)^2 - x^3 - 8 = 0; b) (x - 1)(x^2 + 5x - 2) - x^3 + 1 = 0

1) Tìm x:
a) 2(x+2)^2-x^3-8=0
b) (x-1)(x^2+5x-2)-x^3+1=0
c) x^2-8x+21x-18=0