Explanation:
7/5 = x/3 cross multiply to get 21 = 5x then x = 21/5 = 4.2
12/1 = 9/x cross multiply to get 12x = 9 then x = 9/12 = .75
x/7 = 6/5 cross multiply to get 5x = 42 then x = 42/5 = 8.4
3/x = 9/7 cross multiply to get 21 = 9x then x = 21/9 = 2.33
2/x = 5/9 cross multiply to get 18 = 5x then x = 18/5 = 3.6