if the line is vertical, then the equation is x = -4
the value of x is -4 and there are an infinite number of values of y that satisfy the equation, (-4,7) being one of them.
consider the equation for the slope of the line.
the slope is equal to (y2 - y1) / (x2 - x1).
if (x2 - x1) is equal to 0, then the slope is undefined.
if (x2 - x1) is equal to 0, then x2 must be equal to x1.
this means that you have a different value of y for the same value of x.
this can only occur if the line is perpendicular.
if the line is perpendicular, then the equation becomes x = c
c can be any constant.
this means that there are an infinite number of possibilities for the value of y.
here's a graph with a couple of examples of points that are on the line.
you can see that there are multiple values of y for the line x = -4, with y = 7 being one of them.