Optics : Lens Combinations with laser beam problem

[Blitzy89]

Hello. I've been struggling with this problem for some time, and honestly I do not know how to solve it.

Homework Statement

Two positive lenses are to be used as a laser beam expander. An axial 1.0 mm diameter beam enters the short focal length positive lens, which is followed by a somewhat longer focal length positive lens, from which it emerges with a diameter of 8.0 mm. Given that the first lens has a 50.0 mm focal length, determine the focal length of the second lens and the separation between the lenses

I suppose that yi=1.0mm; yf=8.00mm;f1=50.0mm

Homework Equations

Focal length of system = f
1/f = 1/f1 + 1/f2
Magnification of image Mt=(yf/yi) (per lens)

The Attempt at a Solution

I tried to solve this by combining the two lenses as a system, but I can't find an answer that makes sense.
Any help is greatly appreciated.
Thank you

 

[mgb_phys]

Do you understand the system before putting numbers in ?

http://www.edmundoptics.com/technical-support/lasers/beam-expanders/