To find the population of bunnies after 5 months, we'll use the given equation \( P = 12(3)^x \), where \( x \) is the number of months since March.
We need to calculate the population for \( x = 5 \):
\[ P = 12(3)^5 \]
Next, we calculate \( 3^5 \):
\[ 3^5 = 3 \times 3 \times 3 \times 3 \times 3 = 243 \]
Now we can substitute that value back into the equation:
\[ P = 12 \times 243 \]
Calculating \( 12 \times 243 \):
\[ P = 2916 \]
Therefore, the population of bunnies after 5 months will be \( \boxed{2916} \).