If you distribute each side of the equation, you’d get 12g+18 = 12g+18
This statement is always true, no matter what you put in place of g.
That means any and every real number will be a solution.
Therefore there are infinitely many solutions.
If the equation had worked out to be 12g+18 = 12g+7 or something else where the variable terms were identical, but the numbers weren’t, then there’d be no solution, since there’s no way to make 18=7.