Processed cheese must be at a pH of 5.5 to 5.9 to prevent the growth of unwanted microorganisms. If sample of cheeese has an [OH^-]of 7.76*10^-9, what does it mean? Show calculation. I really need help with please. Kindly show me to compute it thanks a lot.

You're over thinking this problem.

(H^+)(OH^-) = Kw = 1E-14
Substitute OH and solve for H^+.
Then pH = -log(H^+) and compare with th range of 5.5 to 5.9