How can i finish? I am quite confused about this ? Would you like to explain it better?
Grandad's reply does not require 'better' explaining. What needs better explaining is your reply. Specifically, what is it that you didn't understand in Grandad's reply. eg. Are you familiar with the area formula that has been used? If not, then how are we meant to know this unless you say so.
By the way, did you bother to draw a diagram of the problem?