Question: It's been a long time since I've done algebra. I remember how to do it, but I'm at a loss to explain it. For instance, my son has the following problem;
$$\frac{3}{Q+1}+\frac{2}{Q}$$ So I say, you just have to find a common denominator (like when you add "regular" fractions). All you need to do is this;
$$\color{red}{\frac{Q}{Q}}\frac{3}{Q+1}+\frac{2}{Q}\color{red}{\frac{Q+1}{Q+1}}$$
What is this called?

Answer: 
You said it: it's called "finding a common denominator".
"Finding the *least common multiple* of both these denominators" and multiply the denominators to get the *least common multiple*.