What is 6a + b - 15

It's supposed to be

A coefficient

There is not enough information on this question

You can't say what that is without knowing what a and b are. Did the question ask what the coefficient of a is? A coefficient is just the normal number that doesn't change (like 7) that is multiplied by a variable (like x). So the coefficient of 6a is 6. Also, b is the same as 1b so the coefficient of b is 1