The sides of a parallelogram follow two important principles. The sides opposite are equal to one another and they are parallel to each other. A side can be calculated using different formulas for a parallelogram. For calculating side $a$ we have the following$\begin{aligned} a &= \frac{p-2b}{2} \\[1em] a &= \frac{A}{h_a} \\[1em] a &= \frac{h_b}{\sin \left( \alpha \right)}. \end{aligned}$The first formula originates from the one for the perimeter, the second formula from the one for the area and the last formula from the one for the height. The formulas for the other side $b$ are symmetric to the ones for the side $a$, i.e.$\begin{aligned} b &= \frac{p-2a}{2} \\[1em] b &= \frac{A}{h_b} \\[1em] b &= \frac{h_a}{\sin \left( \beta \right)}. \end{aligned}$