Answer
The area of a rhombus can be calculated : $A = a\times a \times sin \theta$.
Work Step by Step
Every rhombus is a parallelogram, so the formula $A=a\times b\times sin\gamma$, applies to every rhombus too. In a parallelogram the two adjacent sides were $a$ and $b$ with the acute angle of $\gamma$. In a rhombus these sides are $a$ and $a$, also the that they form acute angle is $\theta$.
So the area of a rhombus can be calculated : $A = a\times a \times sin \theta$
