Question

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

Answer 1

You can rewrite this expression so that you can view it as a factored difference of squares.

If \((x-y)(x+y)=x^2-y^2\), then
\[(\color{red}{a+b}-c)(\color{red}{a+b}+c)=(a+b)^2-c^2\]
What remains is to expand \((a+b)^2\).