Let width be x
Area: 15√2
Diagonal; 4x
First, you need to find the length using pythagorean theorem.
a=x
c=4x
b=?
Solve this in terms of x and you will find the solution to be; √15x²
Knowing this, plug the number back into the area formula to find the value of x,
(x)(√15x²)=15√2
...
..
x⁴=30
Length= √(15√(30))
Do the calculations just to be sure
Hope I helped :)