$(2x+3) + (5x-7)=$
$2x+3+5x-7=7x-4$
$(2a^{2}+3a-6)+(-a^{2}-3a+9)=$
$2a^{2}+3a-6-a^{2}-3a+9=a^{2}+3$
$(4y-7)-(9-2y)=$
$4y-7-9+2y=6y-16$
$(2ab^{2}-9a^{2}b-3ab)-(2ab-5a^{2}b-8ab^{2})=$
$2ab^{2}-9a^{2}b-3ab-2ab+5a^{2}b+8ab^{2}=10ab^{2}-4a^{2}b-5ab$
$(5x^{2}-(8x^{2}-7x)-(2-9x)=$
$5x^{2}-8x^{2}+7x-2+9x=-3x^{2}+16x-2$
$-(m-1)-(2m+n)-3n+(4m-5)=$
$-m+1-2m-n-3n+4m-5=m-4n-4$
$5x-[2x-(7x-4)]-4=$
$5x-[2x-7x+4]-4=5x-2x+7x-4-4=10x-8$
$2x-{3x-[4x-(5x-1)]}=$
$2x-{3x-[4x-5x+1]}=2x-{3x-4x+5x-1}=2x-3x+4x-5x+1=-2x+1$