What is the sum of 12 5i and 3 4i?

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