So x 2x are the sides using Pythagorean theorem x^2 + (2x)^2= 5x^2
So the side is the square root of 5x^2 so now we have the third side
The sum of the sides should add to 28 so:
X+2x+√5x=28 factor the x
X(3+√5)=28 and we know √5=2.236
So x= 28/5.236= 5.347