The Exact Homology Sequence (Exact Sequence of Triples)

Problem:

Let X = X_1  /  X_2, and A = X_1  /  X_2.  Using the exact sequence of triples, show that if the inclusion (X_1, A) --> (X, X_2) induces an isomorphism on homology, then the same holds for the inclusion (X_2, A) --> (X, X_1).

Notation:
X_1 is X subscript 1
/ is union
/ is intersection
--> is an inclusion map
H_q (X, A) is the quotient module, the qth relative homology module of X mod A

Need a step-by-step proof outline.