2 steps to this problem.
1) Figure out the length of the sides. The perimeter is:
2L + 2W
We know that the width is

of the length, so we can rewrite this to be
2L + 2(

)W
or in other words,
3.5L = 70
-->
L = 20
-->
W = 15
2. Figure out the length of the diagonal. The shape is a right triangle, so you can use the Pythagorean Theorem here.

-->

-->

= diagonal length = 25