Question 1112654
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The form of the equation of a parabola that opens up or down that I like to use is this:
{{{y-k = (1/(4p))(x-h)^2}}}<br>
In this form, the vertex, is (h,k), and p is the distance from the vertex to the focus and from the vertex to the directrix.  With that definition for p, 4p is the length of the latus rectum.<br>
In your problem the vertex and the length of the latus rectum are given, so you just need to plug in the values -- remembering that the coefficient 1/4p is negative because the parabola opens downward.<br>
Answer: {{{y-2 = (-1/20)(x-4)^2}}}