Question (a):
We're asked to write (in interval notation) where the function is decreasing.
We can see that the function decreases from #x = -4# to #x = 2# (because it is increasing on either side of this region). Thus, in interval notation this is
#(-4,2)#
Question (b):
#f'(-3.9)# will be negative, because it occurs after the local maximum at #x = -4# (the function is decreasing during this interval, so its derivative is negative)
#f'(-4.1)# will be positive, because it is located before the local maximum (the function is increasing during this interval, so its derivative is positive)