Need help with (b). Need answer as a number. This is a time series analysis question. Not sure how to handle upside triangle with subscript.

Table 1 : Time Scrims I Ii 12 IT lit Lime* 7 10 / 11 15\\ Iti 21 1 BT 71 | lil lib Till | 71| {}| 5.) (12\\ 77 TU }ti 74 Evaluate each of the following expressions ( i.e., five a numerical answer ! " ( b ) ( 3 points ) ViaIT + Vast Val's