Salut !
A = 3x²-(x²-4x+5)-4x(x-3)
= 3x²-x²+4x-5-4x²+12x
= -2x²+16x-5
B = (2x²-7x+4)-4x(-3+2x)
= 2x²-7x+4+12x-8x²
= -6x²+5x+4
C = 5x(-3x-6)-(-15x²+18x-6)
= -15x²-30x+15x²-18x+6
= -48x+6
D = (x-5)(3x+5)+(4x-2)(5x-2)
= 3x²+5x-15x-25+20x²-8x-10x+4
= 23x²-28x-21
E = (4x-5)(2x-5)-(4x+1)(2x-3)
= 8x²-20x-10x+25-8x²+12x-2x+3
= -20x+28
F = (3x+2)(2x-5)-(6x-5)(4x+2)
= 6x²-15x+4x-10-24x²-12x+20x+10
= -18x²-3x