The width of a rectangle is 1 less than twice its length. If the area of the rectangle is 40 cm, what is the length of the diagonal?

Note: Your answer must be a number. It may not contain any arithmetic operations.

The length of the diagonal is....

I need help with this problem :/

the width is = 2L - 1

length is L

so.... (2L-1)(L) = 40

2L^2 - 1L = 40

2L^2 - 1L - 40 = 0