MCAT Practice question

on one of the practice problems amcas gave the equation /0 + 1/i = S and asked to solve for the strength of the person's eye. Ive never seen this equation anywhere before.  Looking back it was probably a plug and chug especially since they gave the equation but it was the first time I have seen this equation. I'm only familiar with the solving for focal length and magnification. Are there any videos on lens formula solving for strength anywhere?