You have 3 variables (A, B, and C): the number of stitches on a baseball, the number of dimples on a golf ball, and the number of yards on a American football field. If the equation A - B = C is true, then which variable must go with the letter A?

A has to be the largest number, which is the number of dimples on a golf ball.

There are 216 stitches on a baseball.
https://wezen-ball.com/how-many-stitches-are-on-a-baseball/#:~:text=There%20are%20216%20stitches%20on%20a%20Major%20League,and%20function%20initiated%20by%20the%20factory%20rolling%20machine.

There are 120 yards on an American football field, 100 if you don't include the end zones.
https://www.dimensions.com/element/american-football-field

The number of dimples on a golf ball is not fixed, but is somewhere over 300.
https://golf.com/gear/golf-balls/how-many-dimples-on-a-golf-ball/

Putting them together, 216 + 120 = 336, and 336 is a possible number of dimples on a golf ball. (It cannot be determined which numbers are attached to B and C - they are interchangeable.)