To determine the missing side lengths we need first notice that this is a right triangle; we also notice that we know and angle and its adjecent leg.
Now, u is the hypotenuse of the triangle; to determine it we can use the cosine function which is defined as:

In this case we know that the angle is 60° and the adjecent leg is 13; then we have:

Now, to determine the remaining length, v, we can use the pythagorean theorem which states that:

where a and b are the legs and c is the hypotenuse.
In this case the hypotenuse is 26 and one of the legs is 13, then we have:

Therefore, we conclude that:
