We know that each generation takes approximately 3 weeks, then, we can estimate how many generations we do have in one year, remember that one year has
Then, in one year we have
In the real world, we can't have half of a generation or a decimal generation, then, let's approximate it to the nearest integer, in that case, 17 generations.
We have the expression that predicts the number of mice, then we can use that equation to find the result for 17 generations:
Evaluate that at x = 17
With an offspring of
And the ending mice
Therefore, the final answer is