text: Here is the rule: you have to have the same number of equations as you have variables to solve.  For example:\n\nx + y = 1\n\nx - y = 1\n\nHere you have two equations and two unknowns.  The process is to solve one equation for one of the variables, and substituting that into the other:\n\nx = 1 - y\n\n(1-y) -y = 1\n\nNow you solve the new equation for y:\n\n-2y = 0\ny = 0\n\nNow you plug y into any of the equations (preferably the easier one), and you get x.\n\nx + 0 = 1\n\nx = 1\n\nOf course this gets more complicated with each new variable and equation, but the process is the same.  You keep substituting until you get one equation with one variable.