In the situation you have there, you can add the two equations together, because all the terms have the same units: units of force. You can never add two physical quantities together unless they have the same units. If you have a situation like yours, however, where everything in sight has units of force, you can use substitution, elimination, Kramer's rule (assuming you have a linear system), etc., to solve a system of equations. It also depends on what you're trying to do.

As for the MCAT math, I'm assuming that algebra-based physics is on the exam. In that case, I would study high school algebra, some geometry, and make sure you master your trigonometry. Trig is important, because in physics you're forever resolving vectors into different coordinate systems, which usually ends up being a trigonometric problem using sines, cosines and tangents.

For the most part, what's really useful in physics is the ability to solve equations with lots of variables for a particular variable or parameter. You've got a long list of knowns, and probably a short list of unknowns: you'll have to be able to eliminate as many unknowns as possible in order to solve for your target variable (the quantity you want to know). You have to be able to look at a problem statement, draw a picture, and then start writing down correct equations that are relevant to that picture.