The function is decreasing when the derivative is less than zero. Using your correct calculation of the derivative:
Now you can analyze the derivative to find out where it is negative. On what interval(s) is it negative?
On what intervals is the function f(x)=x3−3x 2
decreasing and concave up
i took the derivative and got 3x^2-6x. Then i found the zeroes which are x=0,2. I checked where the function was decreasing and i got (-infinity,1] but it keeps saying its wrong. I don't get why.