How do I approach simultaneous equations with 2 unknowns?

The two common ways to approach them is either by substitution or addition/subtraction. Firstly, for substitution, rearrange one of your equation so that one of the unknowns becomes the subject and then substitute this into the second equation. Then solve that unknown and substitute back in to the original equations to find the other unknown.
Secondly, for addition or subtraction, either subtract or add the equations together in such a way that one of the unknowns are removed. This will leave one unknown so that it can be solved. Once solved, substitute back into the original equations to get an answer for the other unknown.