The graph of the function y= x3+6x2+7x-2cos x changes concavity at x=

Answer 1

Concavity changes sign where the second derivative is zero. That would be where
6x + 12 + 2 cosx = 0, which can also be written
3x + 6 + cosx = 0

This transcendental equation will have to be solved numerically by iteration.
If x = -2, 3x + 6 + cosx = -0.42
If x = -1.9, 3x + 6 + cosx = -0.02
-1.9 is close to the correct answer