Use elimination and subtitution method to solve the problem.
First, eliminate x and you'll find the value of y
x - 4y = 12
x - y = 0
--------------- - (substract)
-3y = 12
y = 12/-3
y = -4
Second, subtitute -4 as y and you'll find the value of x
x - y = 0
x- (-4) = 0
x + 4 = 0
x = -4
The solution
x,y = -4,-4
