3 times thre larger of 2 numbers is equal to 4 times the smaller. The sum of the numbers is 21. Find the numbers.

let L = larger and S = smaller

3L = 4S

L+S = 21

L = 21-S

3(21-S)=4S

63 - 3S = 4S

7S=63

S= 63/7

S= 9

L = 21-9 = 12

so the numbers are 9,12

Check:  3*12 = 4*9      36 = 36 correct