Can someone write a proof for this

Let a,b,n be any integer where n is > or equal to 2. Prove that is a is congruent to b (mod n) than a^2 is congruent to b^2 (mod n)

Suppose
a-b = kn
is a integer multiple of n.

Then
a^2 - b^2 = (a-b)(a+b) = k(a+b)n
is an integer multiple of n.