The sum of two numbers is 30 in the difference is 12 find The two numbers

Let's call the numbers x and y:

their sum is:

x+y=30

and their difference is x-y=12.

from the first equation: x=30-y

and we insert this into the second equation:

30-y-y=12.

we calculate:

30-2y=12

30-12=2y

18=2y

9=y

so the first number is 9 and the second is 30-9=21.