See my comments in red.
Here's a summary of how you can prove two triangles congruent.
So, all you need here is to say why:
and you can then say that the triangles are congruent (SAS). Can you do this now?
Here's something on equal angles formed by parallel lines.
Because the triangles are congruent, . Can you see why this is sufficient to prove that ?
One of the properties of a rhombus is that its diagonals are perpendicular, which you'll find on this page. Can you see why this is enough to show that ?