y'(x)=9x²-16=(3x+4)(3x-4)
You just have to find the sign of y'(x)
y= 3x^3 - 16x + 2
a) increasing for x < -2.31 and x > 2.31; decreasing for -2.31 < x < 2.31
b) increasing for -2.31 < x < 2.31; decreasing for x < -2.31 and x > 2.31
c) increasing for x < - 4/3 and x > 4/3; decreasing for -4/3 < x < 4/3
d) increasing for -4/3 < 4/3; decreasing for x < -4/3 and x > 4/3
and what window did you use?