It means find the difference between Tn and Tn-1. Tn and Tn-1 are consecutive terms. n = 162. Tn-1=T161=13563. The difference is 164. That is the number you are are supposed to add to Tn-1 to get Tn. Alternatively, you know the first differences of a quadratice sequence form an arithmetic sequence. The number that must be added belongs to this arithmetic sequence. Since n = 162 for quadratic sequence, n = 161 for this number in the arithmetic sequence. T161=4+(161-1)(1)=164.
