Locate the longest side of the right triangle. Label this side the hypotenuse.
Locate the lines that have endpoints that are also the vertex of the angle you are trying to measure. If the line is not the hypotenuse, label this line the adjacent side.
Label the remaining line in the right triangle the opposite side. Note that the opposite side of an angle does not have an endpoint that is the vertex of the angle you are trying to find.
Divide the length of the opposite side by the length of the adjacent side. Enter this number into a calculator, and use the inverse tangent function (also known as "arctan function") to display the measure of the angle. Use the calculator at the website Online Calculators (see the link in the Resources section) if you don't have a scientific calculator of your own.