It is given that OA=a and OB=b. Find OR in each of the following cases:

a) R is a point on the straight line AB such that AR : RB=1:2

b) R lies on the extended straight line AB such that AR:RB= 4:3

c) R is the midpoint of the straight line AB

I solved (a), but I don't know what is the mean of 'extended straight line'