1.
8x^2-20x+6x-15-(6x^2-8x+9x-12)-(6x^2-4x+3x-2)=
8x^2-14x-15-6x^2+8x-9x+12-6x^2+4x-3x+2=
-x^2-14x-1
2.
25x^2-2*3*5x+9-(16x^2-2*2*4x+4)+9x^2+2*3x*4+16 =
25x^2-30x+9-16x^2+16x-4+9x^2+24x+16=
18x^2+10+25
c.
2x-y=6
4x^2-y^2=30
(2x-y)(2x+y)=30
=> 2x+y=30/(2x-y)=30/6=5
5+25+125+625=780