12x - 3y = 9 --- (1) y - 4x = -3 --- (2)
y - 4x = -3
-4x + y = -3
-3(-4x + y) = -3(-3)
12x - 3y = 9 --- (3)
Notice that (1) and (3) are the same.
This means that we have two variables and one unique equation.
Hence, any real value of x and real value of y is a solution, i.e., there are infinitely many solutions.