Every equation can be represented as a graph on X-Y coordinate plot.
For this problem we do not actually need to plot a graph, but having understanding of where a graph lies on coordinate system help in better understanding.
So before reading the explanation I would suggest to google some simple graph of \(x^2\), \(-x^2\), \(x^2\)+1 , and \(x^2\)+x+1 etc
(You can also copy paste same equation on Google ; see attached graph)
Now from figure i.e. understanding of equation, this is an equation of downwards open parabola. As its parabola it
can cut x-axis at two point
When it cuts x-axis its y coordinate will be 0.
So putting value in Y=0
0 = -\(x^2\) + 16x + 36
x = -2 or 18
After both this point 'restorative power' i.e. Y coordinate gets negative. So -2 or 18 is Ans
But Sleep hours practically could not be -2. So correct ans is 18
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Attachments
2019-07-15 21_55_28--x^2 + 16x + 36 - Google Search.jpg [ 25.74 KiB | Viewed 149 times ]
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Akshay, New Delhi, India.