Step-by-step answer:
Assuming you would like to have the different ways to express the given expression.
(a+b-c)(a+b+c)
=( (a+b) - c) ( (a+b) + c) expand by difference of squares
= (a+b)^2 -c^2 or expand completely
= a^2+2ab+b^2 -c^2
