How do you solve this problem using substitution or elimination?

Three times the larger of two numbers is equal to 4 times the smaller. The sum of the numbers is 21. Find the numbers.

Respuesta :

3x=4y
x+y=21

x = 21-y
3(21-y)=4y

63-3y=4y
63=7y
y=9

x+y=21
x+9=21
x=12

answer is (12, 9)