a) 2(x+y)(3x+2y-4) = 2(3x²+2xy-4x+3xy+2y²-4y) = 6x²+4xy-8x+6xy+4y²-8y = 6x²+4y²+10xy-8x-8y
b) (x+2)(x+3)(x+4) = (x²+3x+2x+6)(x+4) = x³+4x²+3x²+12x+2x²+8x+6x+24 = x³+9x²+26x+24