Here to find the diagonal, you would simply split the square diagonally into 2 triangles and use the Pythagorean theorem which is used on triangles:
a² + b² = c²
a and b are both short sides and c is the diagonal side. So since it's a square, a and b would both be 5 and you would solce for c which is the diagonal:
a² + b² = c²
5² + 5² = c² square the 5s
25 + 25 = c² add the 25s
50 = c² square root both numbers to get rid of Cs 2nd power
√50 = √c²
√50 = c you can reduce the square root if you want but √50 should be fine.
√(25 × 2) = c
√(5×5×2)
5√2 = c this is the reduced root