What is the power of the lens in diopters?

[KVM]

Homework Statement

It is desired to magnify reading material by a factor of 3.5× when a book is placed 9.5 cm behind a lens.
What is the power of the lens in diopters?

Homework Equations

1/f = 1/di + 1/do
M = -di/do
P=1/f

The Attempt at a Solution

I set equal the magnification to -di/do and solved for di
3.5=-di/9.5
di = -33.25 cm
Then I plugged into equation to find 1/f which = P
1/f = 1/di + 1/do
1/f = 1/-33.25 + 1/9.5
1/f = 0.075
I don't understand why this isn't right??

 

[kuruman]

What are the units of f in 1/f when you want 1/f to be in diopters??

 

[Gene Naden]

(If 1/f is in diopters, f is in meters).

The magnification is negative; di is positive. If you do that, it should work out.

 

[rude man]

Go with Gene N. but also pay attention to what kuruman said. Together they have the answer!