this is all about the complementary and supplementary angle..

These are the problems I can't solve.. Can anyone explain how to solve this? Advance thanks..

18. If the measure of an angle is (7x-13)degrees and the measure of it's supplement is (5x-11)degrees, find x.

19. Find the measure of the smaller angle of complementary angles, if the measure of the larger angle is (5x+18)degrees and the measure of the smaller angle is half of the larger.

21. Two complementary angles have the difference of 18degrees. Find their measures.