Answer
$a.\quad x=-0.82,\ x=1.66$ and $x=5.16$
$b.\quad x\in[-0.82,1.66]\cup[5.16,\infty)$
Work Step by Step
$a.$
Graph $f(x)=5x^{2}-x^{3}$ (red on the image)
and $g(x)=-x^{2}+3x+4$ (blue).
The solution (if it exists) is the x-coordinate of the intersection.
Solution: $x=-0.82,\ x=1.66$ and $x=5.16$
$b.$
Using the graphs of part (a), we see that
the red graph is below or intersecting the blue graph on intervals
$[-0.82,1.66]\cup[5.16,\infty)$
