Write the vertex of the following function as $\left( x,y \right)$ :

$f\left( x \right) ={ -4x }^{ 2 } -12x + 9$

Find $x+y$

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Check out the set:
2016 Problems
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The answer is 16.5.

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The maximum value occurs when $x=\dfrac{-(-12)}{2 \times (-4)}=-1.5$

Substituting this value into $f(x)$ we get the maximum value $y=18$

Hence we get $x+y=\boxed{16.5}$