gives or which is known to be negative since and .
If you look at the sign of f'(x), you see that it is positive before , negative between and 0, and then positive again after zero. This comes from the fact that the coefficient of is positive This implies that the function is increasing, decrease and then increasing. To sketch your graph, compute f at zero and at and then mark the points you got as local maximum and local minimum. Now draw the graph following the variations of f.
For the inflection point, . Setting this to zero gives . Plugging this into f gives the coordinates of the inflection point.
hope this helps