An intramural isomorphism

Suppose the diagram below is part of a double-complex in an abelian category and is vertically exact at $B$; i.e., $\ker (f)=0$:

Use the Salamander Lemma to prove that ${}^{◻}A\simeq A^{\text{vert}}$ and $A^{\text{hor}}\simeq A_{◻}$.