How to Solve Equations?

Consider the equations like x+y=3 and y=3 , then substitute the value of y in the equation and get the value of x.This equation is simple equation. If the equation is quadratic then it will be like x^2 + y^2 = 4. and x^2 -y^2=6.Add this two equations so it will be 2x^2=10 , x^2=5 , so x= square root of 5.So solving the equation is try to find the values of the variables in the equation.