An equation in two variables X and Y is linear if the exponent of both of them is one. But, be careful "one over X" is "X raised to the negative one", the "square root of X" is "X raised to the one half". The solution of a system of two linear equations on two unknowns X and Y is the set of values of X and Y that satisfy both equations at the same time. That is, the set of points (X,Y) on the plane that satisfy both equations.
If the system of equations has three variables, the solutions would be points on the three dimensional space. To solve a system of equations with three variables, by substitution we just need to solve one of the equations for, say, X, and substitute that expression in the other two equations which would produce a system of equations on Y and Z and then solve that system by substitution also.