You would add all the sides together.
It is A because y=20x-5 and so it goes up and to the right
12.74 is the answer to the question
A right triangle contains a right angle, so it must have two sides that are perpendicular. If the slopes of two sides can be shown to be negative reciprocals of each other, then it can be concluded that a right angle is formed. If a triangle contains a right angle, then it is a right triangle.