So think the inequality symbol is like a equal sign.Now since 72 and 105 are like terms you combine them so you subtract 72-72=0 and you do the same thing to the other side so 105-72=33.Now rewrite -3x<33.Then you must make -3 by its self to find x so you divide both sides.-3/-3 is 1 and 33/-3 which is -11 eleven because when you divide or multiply with one negative the answer will always be negative.So rewrite and your answer should be x>-11.
P.S.Anytime you divide on both sides with a inequality the sign is flipped.