Hello there. To solve this question, we'll have to remember some properties about right isosceles triangles.
Given the following triangle:
Notice this triangle has angles 90º, 45º and 45º, hence it is a right isosceles triangle.
In fact, when the angles are equal, the opposite sides to these angles have the same measure, which means that we'll get the measure for side B as
Now, to find the measure of side A, we simply apply the Pythagorean theorem, that says
For a right triangle with legs measuring a and b and hypotenuse measuring c, the sum of the squares of a and b is equal to the square of c,
In our case, a = b = 5sqrt(2), hence
Therefore the length of the side A measures 10.