To unravel programs of equations by substitution simplify the equation to isolate one variable. It does not matter which one however let’s use x. Subsequent, substitute that equation into the variable x of the second equation and clear up for the opposite variable, on this case y. Upon getting y, simply plug it in to both equation and clear up for x.
Fixing Techniques of Equations by Subs.: http://www.shmoop.com/linear-equation-systems/substituting-linear-equation-systems.html