An overhead projector lens is 32.0 cm from a slide (which is the
object). You want a magnification
of -12.0. What focal length should
you use?
pls help?

34.9 cm

Explanation:

You are given the following parameters;

Object distance U = 32 cm

Magnification M = - 12.0

According to formula for magnification;

M = V/U

Where V = image distance.

Substitute V and M into the formula

-12 = V/32

Cross multiply

V = -12 × 32

V = - 384

You can use the formula

1/f = 1/V + 1/U

Where f = focal length

Substitute V and U into the formula

1/f = - 1/384 + 1/32

Find the lowest common factor of the denominator at Left hand side

1/f = ( -1 + 12 ) / 384

1/f = 11/384

Reciprocate both sides

F = 384/11

F = 34.9 cm

He should therefore use the focal length of 34.9 cm

does anyone know or what?

there are no contact forces mentioned on the list of choices that you've provided.