In order to calculate the the side length x we would have to use the pythagorean theorem.
pythagorean theorem
According to the picture, the a side is 10, the b is x, and the c side is 20.
Therefore, using pythagorean theorem we would be able to calculate the value of x.
So:
if a^2 + b^2=c^2
10^2+x^2=20^2
x^2=400-100
x=√300
x=17.3
The side length of x would be 17.3