You first add 5 and 7 which gives you 12. Then, since two supplementary angles add up to 180, you divide 180 by 12 which gives you 15. Next, since 7 is the larger of the two, multiply 7 by 15 giving you 105. To check your work, subtract 105 from 180 giving you 75. Next, put this as a fraction- 75/105 and simplify giving you 5/7 which can also be written as 5:7.
