Only sum may cross #1

One fine morning Harlow went to visit his cousin Plado who lived in a valley on the other side of the Blue Hills. Harlow knew that there were two bridges to cross. Sometimes nasty trolls placed gates on the bridges. Harlow hoped that today was not one of those days. After an hour of walking he came to the first bridge that went across a deep canyon. Harlow did not like what he saw, a gate made of iron bars. It blocked the way across the bridge. Harlow came closer. The gate had a padlock with six holes laid out in the shape of a triangle.

On the ground were six clay balls lying in a row. The first ball had 1 marked on it; the second had 2 on it, the third had 3, the fourth had 4, the fifth had 5, and the last ball had 6 marked on it. There was a message tacked to the gate. It read:

The gate will open to those who can add to nine. Put one ball in each hole in a special way. The sum of the three numbers that are on each side of the triangle must always equal 9.

Harlow started to think.