Review of Quadrilaterals
Read this chapter, which summarizes all properties of various quadrilaterals, including the properties of their diagonals.
Solving for Unknown Values
Solve for 
(picture not drawn to scale).
This quadrilateral is marked as having four congruent sides, so it is a rhombus. Rhombuses are parallelograms, so they have all the same properties as parallelograms. One property of parallelograms is that opposite angles are congruent. This means that the marked angles in this rhombus must be congruent.
