How to find the equation of a parabola given the vertex and one point?

First, write down y=a(x-the x coordinate of the vertex)^2+the y coordinate of the vertex)
second, plug the x coordinate and coordinate of the given point in the above equation to find out a
For example, vertex is (1,2), the point is (3,4)
y=a(x-1)^2+2 
then plug in x=3, y=4 in the above equation to find out a
4=a(3-1)^2+2
4=4a+2
4a=2
a=1/2
the equation of the parabola with a vertex at (1,2) and passes through the point (3,4) is 0.5(x-1)^2+2

The answer is 109 times per second