Solve for x and y

x + y = 3   

2x + 2y = 6

x = 3-y    using equation (i) the value of one variable, say x in terms of the other variable, i.e., y.

2(3-y) + 2y = 6    Substituting value of y in equation (i) to reduce it to equation in one variable

6 – 2y + 2y = 6 or 6 =6   (Try to Obtain the solution for y)

Since we obtain a true statement involving no variable, then the original pair of equations has infinitely many solutions.