One leg of a right triangle is twice as long as the other. write the length of the hypotenuse

Let one of the legs be called x. 

The second leg= 2x 

You know that; c²=a²+b²

c²=(2x)²+(x)²
c²=4x²+x²
c²=5x²
c=√5x²

Therefore, the length of the hypotenuse should be √5x².

Hope I helped :)