With it being an isosceles trapezoid it can be redrawn as a rectangle with the same diagonals. Try whacking a triangle off of one of the ends and sticking it on the other end in a way that makes the trapezoid a rectangle then by the Pythagorean theorem you will find the diagonal to be the sqrt(41). Should be right assuming I didn't break any rules.
This is what I mean. The diagonal that I marked is unchanged and they are obviously going to be the same so they are both unchanged.
Then you get a 5x4 rectangle or two right triangles with a base of 4 or 5 and then you can use the Pythagorean Theorem to solve for the hypotenuse of the triangles or diagonals of the trapezoid turned rectangle.