elementary number theory - Prove $gcd(ka,kb) = k*gcd(a,b)$

For all $k > 0,\ k\in \Bbb Z$ . Prove
$$\gcd(k*a,\ k*b) = k *\gcd(a,\ b)$$

I think I understand what this wants but I can't figure out how to set up a formal proof. These are the guidelines we have to follow