Well, to find the measurement of the unknown side of the triangle, you have to know the formula. I know there is a name for it, but I just can't remember it right now. I know the formula pretty well though. It's a^2 plus b^2 equals c^2. The numbers that are already shown for the right angle and the base are plugged in to replace a and b. C represents the unknown side. For example, if the triangle had the measurements of 5 cm for the right side, and 2 cm for the base, it would be 5^2 plus 2^2 equals c^2. Once you get the answer from those numbers, you find the square root of it and that is the measurement of the hypotenuse.
This lesson was simple for me once I got the hang of it. All you really need to know is the formula, and where to plug in the numbers. What was confusing for me was when the hypotenuse measurement was given, and the right angle or base measurement was unknown. It was kind of difficult for me at first, but I got the hang of it. You have to use inverse operation to find the unknown side.