I am stuck with this topic which I just cant understand , I just know how to take derivative of an equation and also to find the critical points , the problem I am having is the increasing and decreasing part , I just cant understand how its done , i know that

f'(x) > 0 (increasing)

f'(x) < 0 (decreasing)

but i just cant do it whenever i put some values I get wrong answers please help me out