First of all, what is your problem exactly? 2 angles a,b are supplementary angles if a+b=180 degrees. And a,b are complementary if a+b= 90 degrees.

Thus it boils down to basic algebra...

18. solve (7x-13)+(5x-11)= 180 for x

19. Solve (5x+18)+1/2(5x+18)= 90 for x and then find the value (5x+18) by plugging in x

20 .....you can do this one.