First you want to find X using the identities given for a 45-45-90 triangle. When comparing the triangles in the video, we find that X must be 6. Next, use the identity to solve for the measurements of the sides of the triangle. In the video example, 6 is the length of the two sides and 6 times the square root of 2 is the length of the hypotenuse. The video is very short and does not explain a lot of things in detail, but that is because the examples are easy to figure out.