Find the value

Using the identity $(p+q)^{2}=p^{2}+q^{2}+2 p q$

$=(a+b)^{2}-c^{2}$

Using the identity $p^{2}-q^{2}=(p+q)(p-q)$

$=(a+b+c)(a+b-c)$

$\therefore a^{2}+2 a b+b^{2}-c^{2}=(a+b+c)(a+b-c)$