cosets of a subgroup

Halp me please !

I need to prove that two cosets aH, bH of a subgroup H of group G are either identical or disjoint.

can someone point me in the right direction ?

if h1,h2 \in H and a*h1 = b*h2, then

a = b*h2*h1^-1 \in bH

implies aH \subset bH