Question

How do I find length of a rectangular prism

Answer 1

W = width (of the base)
l = length (of the base)
B = area of the base =  wl
h = height (of the prism)
V = volume of the prism = Bh = whl
SA = surface area of the prism = 2wh + 2wl + 2hl

I can't really tell you how to find the lenght of a rectangular prism without knowing what other properties of the prism you already know. These formulas are enough for you to figure it out for yourself.
Try not to ask vague questions next time if you want better answers.

Answer 2

You use the information you're given, along with everything you know about rectangular prisms, to relate the length to the known information.