Kiyoshi can paint a certain fence in 3 hours by himself. if red , the job takes only 2 hours. how long would it take red to paint the fence by himself ?

Kiyoshi paints  1/3  of the fence each hour.

When the friends work together, they paint  1/2  of the fence each hour.

So Red must be painting  (1/2 - 1/3) = 1/6  of the fence each hour.

The rate of  1/6 fence per hour means  1 fence per  6 hours  for Red alone.