The angles in a parallelogram follow two important principles. The angles opposite are equal to one another and the sum of the two angles next to each other measures $180 \degree$, or $\pi$ in radian. This means if one angle is given the other angle can be computed with the following formula$\begin{aligned} \alpha &= 180 \degree - \beta \left( = \pi - \beta \right) \\[1em] \beta &= 180 \degree - \alpha \left( = \pi - \alpha \right). \end{aligned}$On the other hand if the other angle is not given then the angle can be computed using the length of the belonging side and the length of the other height known in the parallelogram. So we have the following formula$\begin{aligned} \alpha &= \sin^{-1} \left( \frac{h_b}{a} \right) \\[2em] \beta &= \sin^{-1} \left( \frac{h_a}{b} \right). \end{aligned}$Important here is that the quotient of other height and belonging side is smaller or equal to $1$, i.e. that other height is smaller or equal to that side ($h_a \leq b$ und $h_b \leq a$). This quotient is equal to the sine of an angle and the image of the sine is $[-1,1]$ which is why this is so important. Otherwise the form is not a parallelogram.