i dont know how to set up the equations

Since the sum of the two numbers is 21,

let x be one number and 21-x be the other.

x + (21-x) = 21

Since "three times the larger of two numbers is equal to four times the smaller"...

3x = 4(21-x)

3x = 84 -4x

7x = 84

x = 12

Since x = 12, 21-x = 21-12 = 9.

Larger number is 12, smaller number is 9.

Note that if you said:

3(21-x) = 4x

63 - 3x = 4x

63 = 7x

x = 9, and 21-x = 21-9 = 12

12 would still be the larger number and 9 would still be the smaller.