There's no real question per say. I've been looking in books and on the internet i can't find anything. I don't know when to use the vertical and horizontal line test to test for injective functions and surjective. Let's say if I was to sketch the graph of x2+4x - 5 I would get a parabola, it's a function cause a vertical line cuts it once. Now a horizontal line cuts it twice... so what does that mean? It's surjective?