Answer: 34.9 cm
Step-by-step 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