You can
put this solution on YOUR website!It is hard to prove without seeing the figure to know where the angles are, but I'll give it a try;
I am assuming <1 and <2 are opposite angles, if they are;
Theorem:
A parallelogram is a rhombus if and only if each diagonal bisects a pair of opposite angles.
Plan: You can use SAS or SSS to find congruent triangles.Then use the congruent parts to help you prove the theorem.
I hope this helps
:)