how to darken left guys face a lot so it will be the same color as the right image?

Brightness/contrast is a very crude tool that I would recommend you just put aside and forget for good. It's useless if you want any sort of control over what you do.

In this case, look at the white point on the dark image. You can immediately see it's very low. So use Levels to lower it on the other. Also tighten the black point the other way:

Then lower saturation to match.