Guo Jiachen's answer to idununderstandasinglething's Secondary 4 A Maths Singapore question.

done {{ upvoteCount }} Upvotes
clear {{ downvoteCount * -1 }} Downvotes
Guo Jiachen
Guo Jiachen's answer
20 answers (Tutor Details)
The other answers already solved your problem.

But if you want to know how to know what rules to use, take a look at this answer.