.
Let ABCD be our rectangle and let O be the intersection point of its diagonals.
We are given that the angle (angles) AOD and BOC are of 60° each.
It implies that the triangle AOD is an equilateral triangle, as well as BOC is an equilateral triangle.
Should I continue from this point or everything is just clear to you ?
If you still have difficulties, let me know through the "Thank you" note.
Then I will help you (at no charge).
In this case do not forget to include the ID number of this problem (1073300) into your message.
Answer. The length of the diagonal is 12 inches.
Good luck and happy learning !