Question 766442
This is a function having infinite number of y-values.  It has a maximum y value.

If you will add and subtract {{{(10/2)^2=25}}}, you can transform into
{{{f(x)=-(x-5)^2-35}}}


The vertex is at the maximum, so all y values are LESS than or equal to (5,-35).

{{{highlight(f(x)<=-35)}}}




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Done using Completing the Square.
Do you need to see the steps?