Since it is a rectangle, that means that all the corners are 90 degrees.
If we take a rectangle and cut it along the diagonal, that is basically two right triangles.
The diagonal is just the hypotenuse of the triangles
To find the hypotenuse
a^2 + b^2 = c^2
18^2 + 11^2 = c^2
324 + 121 = c^2
445 = c^2
c = sqrt. (445)
Hope this helps!