(5x-4)²-(3x-7)²-(4x-5)-6(7x-9)=
25x²-40x+16-(9x²-42c+49)-(16x²-40x+25x)-42x+54=
25x²-4x+16-9x²+42x-49-16x²+40x-25-42c+54=
0-4=
4
