Three times the larger number of two numbers is equal to four times the smaller. the sum of the numbers is 21. find the numbers

X=larger number
y=smaller number

3x = 4y
x+y=21

3x=4y
3x÷3= 4y÷3
x = 4/3y

x+y=21
4/3y+y=21
1 4/3y=21
1 4/3y ÷ 1 4/3=21 ÷ 1 4/3
y = 9

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

Check
3x =4y
3(12) = 4(9)
36=36

C. expression because any expression or equation may contain more than one variable

The answer would be -3.9> -4.5