Question 353349
Use the vertex form, {{{y=a(x-h)^2+k}}}, where (h,k) is the vertex.
The value of a determines the direction of opening and also the width of the parabola. 
When {{{a<0}}} then the parabola opens downwards.
When {{{abs(a)<1}}}, then the parabola is wider than {{{y=x^2}}}.
Let {{{a=-1/2}}}
Then 
{{{y=-(1/2)(x-0)^2+84}}}
{{{highlight(y=-x^2/2+84)}}}
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Here is the graph of the function (in red) with {{{y=-x^2+84}}} (in green) graphed for reference.
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{{{drawing(300,300,-10,10,-5,95,grid(1),circle(0,84,0.32),graph(300,300,-10,10,-5,95,-(1/2)x^2+84,-x^2+84))}}}