Solution:

Jason has 21 balls and Glenn has 15 balls.
If Jason would give Glenn 3 balls, they would have an equal number (i.e., 18 balls each).

If Glenn would give Jason 3 balls, then Jason would have 24 balls and Glenn would have 12 balls.
Thus, Jason would have 2 times as many balls as Glenn.

Detailed Solution through Equations:

If Jason would give Glenn 3 balls, they would have an equal number.
This implies that Jason has 6 balls more than Glenn.

If Jason has j balls, then Glenn has (j − 6) balls.

If Glenn would give Jason 3 balls, then Glenn would have ( j − 9) balls and Jason would have (j + 3) balls.
Now, Jason would have 2 times as many balls as Glenn.
This may be written in the form of the following equation.

j + 3 = 2 (j − 9)

The equation may be solved as follows.

1 j = 3 + 2 × 9 = 21 ; or
j = 21 / 1 = 21.

Food for thought:

Can you solve this puzzle without writing any equations?