Calculating the volume of a cylinder.

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Let $V = \{(x, y, z): x^2 + y^2 ≤ 4$ and $0 \le z \le 4\}$ be a cylinder and let $P$ be the plane through $(4, 0, 2)$, $(0, 4, 2)$ and $(−4, −4, 4)$. Compute the volume of $C$ below the plane $P$.

I have absolutely no idea how to start this question. I was thinking using Cavalieri’s Principle but I'm not quite sure how to or if there's another way of working it out. Please help.

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the normal of the plane is given by taking the cross product of the first point minus the second times the first point times the third. It is i+j+6k. Then the equation of the plane is given by equating the dot product of the normal times the diference between (x,y,z) and the first point with 0. Is is z=(16-x-y)/6=(i6-rcosa-rcosa)/16. Now the volume is given by taking the double integral in polar coordinates over the region 0<=r<=1 and 0<=a<=2.pi of (i6-rcosa-rcosa)/16.