You assume that <i>a</i> is an element of A (that is, <i>a</i> satisfies 
the 
condition
that defines A). Use this assumption to show that<i> a </i> satisfies the
condition that defines B. Then assume that<i> b</i> is an element of B 
(that is,
<i>b</i> satisfies the condition that defines B). Use this assumption to 
show
that <i>b</i> satisfies the condition that defines A.