Afirst number plus twice a second number is 9. twice the first number plus the second totals 21. find the numbers

Let x and y be the numbers;
We have x + 2y = 9 and 2x + y = 21;
Then, x = 9 - 2y;
Finally, 2(9-2y) + y = 21 ;
18 - 4y + y = 21;
18 - 3y = 21;
-3y = 3;
y = - 1;
x = 9 - 2× (-1);
x = 9 + 2 ;
x = 11;

Well if you have 20 bats you probably want 20 balls? you don’t have enough context to answer the question.