18. Efectuaţi calculele:
a) (x + 2)(x²-x+3)-(x + 1)(x²+2x - 5);
b) (2x + 1)(x² - 2x+4)-(x-3)(2x² + x + 3);
c) (x-3)(x²-3x+2)-(x+3)(x² - 2x + 3);
d) (2x-1)(-x²+x-2) + (2x-3)(x² − x + 1)