What Focal Length is Needed to Project a 35mm Slide onto a Large Screen?

[scottatkins]

Homework Statement

A 35mm slide(picture size is actually 24 by 36 mm)is to be projected on a screen1.80m by 2.70 m placed 7.50m from the projector. What focal length lens should be used if the image is to cover the screen?

Homework Equations

the only equation i can think of is the lens equation which is (1/Do)+(1/Di)=1/f

The Attempt at a Solution

I have tried starting this problem numerous times and just have no clue how to start it

 

[ehild]

You need the formula for magnification, too.

ehild