undefined slope means slope = infinity or negative infinity, or a vertical line
a vertical line means the x coordinate never changes, it's always -3
so the line is x=-3
If you had tried to use the point slope formula
y=mx+b m= infinity, or negative infinity, but there is no y-intercept. the line never crosses the y axis. you can't say b=0 either. As the line doesn't cross at y=0.
b is undefined with no possible value. Solving for b you get a null set. . y is any value, from negative infinity to positive infinity. y is undefined, but can take on any value and does.
You really can't use the point slope formula when m is undefined, y is undefined and b is a null set