To solve a function like this, simply put the numerical value in place of the unknown variable for which you have to find the solution, for instance '-3' in this case.
g(x) = 2x-2
so to find g(-3), we just need to replace the x in the given expression by '-3'. Working is shown below:
g(x) = 2x-2
putting '-3' in place of x
g(-3) = 2(-3) - 2
g(-3) = -6 -2
g(-3) = -8