1)=2a-a^2-a^2-2a=-2a^2

2)=-3a^3+a^2

3)=x^2+7x-x-7

4)=18x-6x^2-2x^2-18x=-8x^2

5)=a^2b^2-4a^4-4b^4+16a^2b^2=17a^2b^2-4a^4b^4

6)=c^2-c+4c-4-c^2=3c

7)=5x-20-(x^2-4x+4x-16)=5x-20-x^2+16