Question

(a + b - c)(a + b + c)

Answer 1

\[a ^{2}+ab + ac + ab + b ^{2} + bc - ac -cb - c ^{2}\]
=\[a ^{2} + b ^{2} - c ^{2} +2ab \]

Answer 2

In your shoes I'd put to use the "difference of squares" formula to solve this problem.

Hope you remember that (w-x)(w+x)=w^2 - x^2 ( a "difference of squares").

Now, let's go back to your posted math problem.

I'm going to insert additional parentheses in the form of square brackets:

([a+b]+c)*([a+b]-c)

Apply the same principle illustrated above (difference of squares):

[a+b]^2 - c^2 is the intermediate result we wanted.

If you like, go ahead and expand [a+b]^2, remembering to subtract c^2 from that when you're done.