Add the two numbers together to find total votes:
3,400 + 15,600 = 19,000 total votes.
Divide votes for by total votes:
3,400 / 19,000 = 0.1789
Multiply by 100:
0.1789 x 100 = 17.89% voted for it.
Subtract the percent that votes for it from 100%
100% - 17.89% = 82.11% voted against it.
The answers probably need to be rounded, so 18% for, and 82% against.