Question 177120
(c^2-6cd-2d^2)+(7c-cd+8d^2)-(-c^2+5cd-d^2)

First, make sure you don't create any errors in signs by changing the subtraction of the last quantity in parentheses to addition.

{{{(c^2-6cd-2d^2)+(7c-cd+8d^2) + -1*(-c^2+5cd-d^2)}}}
{{{(c^2-6cd-2d^2)+(7c-cd+8d^2)+(-1)(-c^2)+(-1)(5cd)-(-1)(d^2)}}}
{{{(c^2-6cd-2d^2)+(7c-cd+8d^2)+c^2-5cd+d^2}}}

Then, since there are no other operations than the addition and subtraction between these terms, regroup them so like terms are together. Like terms have the same variables raised to the same powers.

{{{c^2+c^2-2d^2+8d^2+d^2-6cd-cd-5cd+7c}}}

Then, you can combine the like terms.
{{{2c^2+7d^2-12cd+7c}}}

These terms are not like terms and so this cannot be simplified.