If im not mistaken, you would use the Pythagorean theorem.
So, its c^2= A^2 + b^2
So, 10^2 = 1x^2 + 3x^2
100 = 4x^2 (You can add the x's since they are both x and both to the power of 2)
I would then use a square root to get rid of that power of two
The square root of 100 is 10, and the square root cancels out on the other side.
10 = 4x
10/4 = 2.5
x = 2.5
I hope that helps!