Examining the diagram, we see that the two triangles have one congruent angle and they share a side, BC. By the Reflexive Property of Congruence, we can therefore claim that this side is congruent between the triangles. Let's add this information to the diagram.
From the diagram, we see that ∠ACB and ∠DCB form a linear pair. This means these angles are supplementary:
m∠ACB+m∠DCB=180^(∘)
Since, m∠DCB= 90^(∘), we can solve for m∠ACB.
m∠ACB+ 90^(∘)=180^(∘) ⇒ m∠ACB=90^(∘)
Let's add this information to our diagram. Note that we have two sets of angles and a non-included corresponding side that are congruent.
