Because the diagonal bisects the the rectangle into two identical right-angled triangles you can apply the Theorem of Pythagorus:
The square of the hypotenuse (diagonal) = the sum of the squares of the length & breadth of the rectangle.
This means:
diagonal (squared) = (26.25 x 26.25) + (56 x 56) = 689.0625 + 3136 = 3825.0625
to calculate the diagonal, you need to find the reverse of 'squared', which is 'square root'
diagonal = square root of 3825.0625 = 61.84709
Note: this is not an exact answer, since the above number is an irrational number with an unending number of decimals, without any pattern. But it is accurate enough considering the number of decimals in the dimensions you supplied.