Which equation represents a vertical line?
(1) y = -x (3) x = y
(2) y = 12 (4) x = 12

The answer is x=12
y=-x would be a negative slope
y=12 would be a horizontal line because you can put any point on that line and it's y would always be 12 but it's x coordinate can be anything else
x=y is a positive slope
x=12 is a vertical line because you can put any point on that line and it's x coordinate would always be 12 but its y coordinate can be anything else