Hello thereddevils(opp sides of a parallelogram)So the triangles are congruent, SAS.
(corresponding angles, )
For (2), a parallelogram is a rhombus if its diagonals are perpendicular. Well, can you see how to use a pair of equal angles in the congruent triangles to prove ? And you're given that , and . So that will be enough to prove is a rectangle, and therefore .
For (3), use the congruent triangles to show that . Also, , to show that .
Can you fill in all the details now?