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 Homework Statement

I'm given the function
f(x)=##2x^3+x^26x2+sin(x)##
 Homework Equations
 MVT or IVT?
1.Prove that f'(x) is strictly decreasing at ( ##\infty##,a) and strictly increasing at (a,##\infty##).
2.Prove that f'(x) has exactly two roots.
I tried to find f''(x)=0, but I'm not able to solve the equation. What should I do?
2.Prove that f'(x) has exactly two roots.
I tried to find f''(x)=0, but I'm not able to solve the equation. What should I do?