What would be the expression for a rectangle with a width of 3x-5 and a length of 6x

We can fish around blindly and offer some information that may help to answer the question that you may plausibly be trying to ask:

- The area of a rectangle is (its length) times (its width).  So the area of the
rectangle with these dimensions is 
                                                     (3x - 5) (6x) = 18x² - 30x.

- The perimeter of a rectangle is (2 lengths) + (2 widths).  So the perimeter of
the rectangle with these dimensions is
                   2(3x - 5) + 2(6x) = 6x - 10 + 12x = 18x - 10.

\( 1)\ \ \ the\ area\ of\ a\ rectangle =the\ width\cdot the\ length\\\\A_r=(3x-5)\cdot 6x=18x^2-30x\\\\2)\ \ \ the\ perimeter\ of\ a\ rectangle=2\cdot( the\ width+the\ length)\\\\P_r=2\cdot(3x-5+6x)=2\cdot(9x-5)=18x-10\\\\3)\ \ \ the\ diagonal\ of\ a\ rectangle:\\\\(the\ diagonal)^2=(the\ width)^2+(the\ length)^2\\\\d^2=(3x-5)^2+(6x)^2=9x^2-30x+25+36x^2=45x^2-30x+25\\\\the\ diagonal= \sqrt{45x^2-30x+25} \)