Recall that the area under a probability curve is always 1.0. Here we have a probability distribution from 10.6 to 18.6 minutes, of length 10 minutes, so the height of the probability "curve" is 1.0 / 10, or 0.1.
To answer this question, find the area under the probability "curve" between 13.8 and 16.8 minutes. The difference between 13.8 and 16.8 is 3 units, so the desired probability is 3(0.1), or 0.30.