Hi,
You can write your problem as:
x = 8 + 3y
x + y = 28
So
(8 + 3y) + y = 28
4y = 28 - 8
4y = 20
y = 20/4 = 5
x + y = 28
x + 5 = 28
x = 28 - 5 = 23
The numbers are 23 and 5.
Hope this helps! If my answer was not clear enough or you’d like further explanation please let me know. Also, English is not my first language, so I’m sorry for any mistakes.