Answer :
You have to remember that:
[tex](-)*(+)=(-)[/tex]
and
[tex](-)*(-)=(+)[/tex]
[tex](11x^2+7)-(6x^2+3x-3)=11x^2+7-6x^2-3x+3=5x^2-3x+10[/tex]
[tex](-)*(+)=(-)[/tex]
and
[tex](-)*(-)=(+)[/tex]
[tex](11x^2+7)-(6x^2+3x-3)=11x^2+7-6x^2-3x+3=5x^2-3x+10[/tex]