Compound Microscope: Solving for Objective Focal Length

[map7s]

A compound microscope has the objective and eyepiece mounted in a tube that is 18.0 cm long. The focal length of the eyepiece is 2.08 cm, and the near-point distance of the person using the microscope is 25.0 cm. If the person can view the image produced by the microscope with a completely relaxed eye, and the magnification is -4350, what is the focal length of the objective? (Include the sign.)

I tried using the equations

d i =d- fo

M = (d i * N)/(fo*fe)

since I could not find d i b/c I didn't know fo, I substituted d-fo for d i in the second equation. When I isolated fo, I got

fo=(dN)/(Mfe +N)

I thought that I was doing this right...what did I mess up?

 

[map7s]

did I do an algebraic mistake or...I assumed that the 18 cm = d...was that a wrong assumption to make?