Question

How do you solve this equation . Q=3a+5ac

Answer 1

A "solution" would be a set of three numbers ... for Q, a, and c ... that
would make the equation a true statement.

If you only have one equation, then there are an infinite number of triplets
that could do it.  For example, with the single equation in this question,
(Q, a, c) could be (13, 1, 2) and they could also be  (16, 2, 1).
There are infinite possibilities with one equation.

In order to have a unique solution ... three definite numbers for Q, a, and c ...
you would need three equations.