You could try
(h,k) is the vertex and (x,y) are the coordinates it passes through.
Use these to find a and you're all set.
Please someone help. I have summer school and I got a Take-Home Quiz. My teacher knows about the whole, google or yahoo the answer thing, so he does not care. Anyways one of the questions is this: Write an equation of a parabola with vertex (-2,3) and passing through (0,15). I have tried to get this equation, but failed. Please help.