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

Sadly, there's no such thing as "the expression for a rectangle", so your
question can't be answered simply and directly. 

We can, however, 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 .