A=x^2-2x-3,B=2x^2-5,C=1/2x^2-7x+6
A-(B-4C)
=A-B+4C
=(x^2-2x-3)-(2x^2-5)+4(1/2x^2-7x+6)
=x^2-2x-3-2x^2+5+2x^2-28x+24
=x^2-30x+26
A=x^2-2x-3,B=2x^2-5,C=1/2x^2-7x+6
A-(B-4C)
=A-B+4C
=(x^2-2x-3)-(2x^2-5)+4(1/2x^2-7x+6)
=x^2-2x-3-2x^2+5+2x^2-28x+24
=x^2-30x+26