Interpolation is when divide 2 xodinates in a certain ratio to get the other odinate. For example suppose you have (2,6) and (8,3) and you want to find the y ordinate corresponding to x=3. 3 divides 2-8 in ratio 1:5 so divide 6-3 in ratio 1:5 and since the line is down subract your answer from the 6.