$\text{a)}\left(3x+2\right)\left(6x-1\right)=18x^2-3x+12x-2=18x^2+9x-2$

$\text{b)}\left(2x+3\right)\left(3x-4\right)=6x^2-8x+9x-12=6x^2+1x-12$

$\text{c)}\left(x+2\right)\left(x-5\right)=x^2+2x-5x-10=x^2-3x-10$

$\text{d)}\left(2x-5\right)\left(-1-4x\right)=-2x-8x^2+5+20x=-8x^2+18x+5$