# Math Help - Commutative Diagram of Modules

1. ## Commutative Diagram of Modules

Let $A$ be a ring. The following is a commutative diagram of $A-$module homomorphisms, and the rows are exact.

Suppose that $f_1$ is surjective and that $f_2$ is an isomorphism. Prove that $f_3$ is an isomorphism.

Attempt
Onto- $f_3 = t \circ f_2$ and both $f_2$ and $t$ are both onto so $t \circ f_2$ is onto.
1-1- This one doesn't seem as trivial. I am a bit confused on showing this.

Thanks in advance for any hints.