how do I write down the values of each letters without measuring it?

Given the graph of parallel lines and a transversal
There is an angle = 90
This means the transversal is perpendicular to the parallel lines
So, All the angles are right angles
So, the measure of all angles = 90
So,
[tex]\begin{gathered} m\angle g=90\degree \\ m\angle h=90\degree \\ m\angle i=90\degree \end{gathered}[/tex]