Median is the center dot in the shaded box (in blue color). Look at the Time (h) axis for their values. For Brand X it is 13 whereas for the Brand Y it is 16
(b) If we compare their median values we can see that the difference is 3 hours (16-13 = 3). It means that the Brand Y battery will last longer compared to Brand X battery as Brand Y lasts for 16 hours whereas Brand X battery lasts for 13 hours.