To find the sum of the complex numbers 12 + 5i and 3 + 4i, we need to add the real parts together and the imaginary parts together.
The real parts are 12 and 3. Adding these gives:
12 + 3 = 15
The imaginary parts are 5i and 4i. Adding these gives:
5i + 4i = 9i
Now, we combine the results of the real part and the imaginary part. So the sum of the complex numbers is:
15 + 9i
Thus, the final answer is:
15 + 9i