The side of the square is approximately 1.7
How to find the side of the square
Drawing a diagonal (BD) on the square will result to similar triangles with proportions as follows
BD / BA = BC / AC
BD / 3 = 4 / 5
BD = 12/5 = 2.4
And diagonal of a square is x√2
x√2 = 2.4
x = 2.4 / √2
x = 1.697
The side of the square is approximately 1.7