>9th grade son needs to learn how to solve this function and
>I'm rusty.

There must be something wrong with your terminology. Functions are not solved, equations are. Functions may be evaluated, simplified, multiplied, but not solved.

>Don't need it solved just a push in the right
>direction please.
>
>f(x)=(-3-10x)/(10x^2 -7-15x), find f(-6)

If that is the question: find f(-6), there is nothing to solved, but just compute f(-6), i.e. evaluate f(x), as defined above, for x = -6.

>I'm thinking that I plug the -6 in for x

Just plug and calculate.

> then solve.

Nothing to solve here.

> Am I correct?

You are correct to mind your son's maths.