According to PEMDAS, you first have to do the multiplication since there isn't any parenthesis or exponents or division. The multiplication part is where it says ab, because those two variables together mean multiply.
More clearly:
15 - a × b
a, as you said, is 2, and b is 3, so you multiply them together to get 6.
15-6 is what's left. You can figure it out easier now!