If that’s meant to be x^2 - 6x - 16
Then factorise to (x+2)(x-8)
So x intercepts are -2 and 8
You should always remember that a cylinder consists of circles that are all over each other to form the height of the cylinder.
So the volume of the cylinder= the area of the surface circle* the height.

Then,

So,

radius is approximately 3.4cm
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Hope this helps.