Question 1033490
Vertex form -

y-k=a(x-h)^2

enter the given vertex -

y+1=a(x-2)^2     (<<< you see why it's y+1.  y- (-1) is y+1)

Now we have the equation with one unknown to solve, "a". So use the given point.

-3+1=a(4-2)^2

-2=a *4

a= -1/2

y+1=-1/2(x-2)^2 

A graph to help illustrate the results 

*[illustration Screen_shot_2016_05_08_at_12.14.31_PM.jpg]