There are many solutions. All straight line graphs are of the form y=mx+c where m (the slope) and c (the y-intercept) are constants.
So -19=-3m+c. Many values of m and c satisfy this equation and we could write this as c=3m-19, making c depend on m.
So if m=6, c=-1 and y=6x-1. If we put x=-3, then y=-19. So one solution would be y=6x-1.
If m=7, c=2 and y=7x+2. When x=-3, then y=-19. Another solution would be y=7x+2.
If m=1, then c=-16 and y=x-16. When x=-3, y=-19. So yet another solution would be y=x-16.
And so on.
But not all graphs have to be straight lines, so there are also many curves that pass through the given point.
y is just a function of x and we could write f(x)=6x-1, or 7x+2, or x-16, or whatever.