a) the derivative of f at 0 is -12, so the normal line has equation y = x/12 + c, and since f(0) = 20, c=20.
b) a line is parallel to the x-axis when it has 0 derivative, so you want to find the points at which the derivative of f is 0. The derivative of f is a quadratic, so just use the quadratic formula.
c) f is a cubic, and from b) you know the stationary points, so sketch it from this. Add on the lines you found in b).
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