Note that f(x) is the product of two linear terms. So it must be a quadratic function. To find the quadratic term we simply expand the product of both terms. We have that

If we expand the product we get

we note that this is of the form

where a is the quadratic term, b is the linear term and c is the constant term. By comparison, we can see that the quadratic term is 15, the linear term is 9 and the constant term is -6. All of this criteria are met by the third option. Meaning the third option is the correct one