Find two numbers if their sum is 3 and their difference is 21 plz

So you make two equations:

x+y=3  because their sum is 3
x-y=21 because their difference is 21

now you have to solve one equation, I'll use the first one

x=3-y

plug in this x into the other equation
x-y=21
(3-y)-y=21
3-2y=21
-2y=18
y=-9

now solve for the other number
x-9=3
x=12

your numbers would be 12 and -9

X + y = 3
x - y = 21    Adding:-
2x = 24
x = 12

y = 3 - x = 3  12 = -9

Answer the 2 numbers are 12 and -9.

Here, feel free to ask if there are any questions