Let us draw out the rectangle in the question so that we can understand it clearly
In the figure( see attached file) we have got the measures of two adjacent sides. We know that the angles of the rectangle are right angle that is 90°
We can see that ACD forms an right triangle
now according to the pythagorean theorem we know that

where c is the hypotenuse of the right triangle
a and b are the two sides of the triangle
Here we are supposed to find the hypotenuse that is AC
now lets plug in the values of a and b
a= AD = 60cm
b= CD = 91 cm
so we have,




c = 109 cm
Hence the diagonal measure 109cm