x^3 -2x^2 - 3x < 0
Let's solve this
x^3 - 2x^2 - 3x = 0 factor
x( x^2 - 2x - 3) = 0
x( x - 3) ( x + 1) = 0
Set each factor to 0 and solve for x and the solutions are
x = 0 , x = 3 and x = -1
We have four possible solution intervals
(-inf, -1), ( - 1, 0) , (0, 3) and (3, inf )
This graph shows that the intervals in red are the correct solution intervals :
https://www.desmos.com/calculator/z8f6gksyuq