Z-Scores and Standard Deviation Help

Hey, I'm new to this forum. I just started studying psychology and I haven't quite grasped the workings of z-scores. In an exercise, I have to answer the following question:

A score that is 20 points above the mean corresponds to a z-score of z = +2. What is the standard deviation?

It's probably a very simple question, but I've blanked out. Could someone please explain to me how the answer is achieved?

Another one I'm confused about... For a population with σ = 12, a score of X = 87 corresponds to z = -0.25. What is the mean for this distribution?

Z-Scores and Standard Deviation Help