Marika is training for a track race she starts by sprinting 100 yards . she gradually increases her distance , adding 4 yards a day for 21 days how far does she sprint on day 21  using an=100+(n-1)4

For this case, you must use the given expression, taking into account that:
 n = number of days.
 We have then that for day 21, n = 21.
 Substituting we have:
 a21 = 100 + (21-1) * 4
 a21 = 180
  
 she sprints 180 yards on day 21

Where are the functions?