How do i find the missing side or hypotenuse in a 45 45 90 triangle

You're given one leg. Because you know both legs are equal, you know the length of both the legs. You can find the hypotenuse by multiplying this length by the square root of 2. Divide the hypotenuse by the square root of 2 to find the legs (which are equal).

There is one equation to remember for a 45-45-90 right triangle.
hyp = sqrt2 * leg
If you are given either the leg or hypotenuse, fill in the one you have to solve for the other.