Algebraic Proofs in Combinatorics $\binom{n + m}{2} = nm + \binom{n}{2} + \binom{m}{2}$

Question

Prove the following identity using an Algebraic Proof.

$$\binom{n + m}{2} = nm + \binom{n}{2} + \binom{m}{2}$$

I have no idea where to begin on this problem or let alone finish it.