Kenzie bought a shirt for $18. the next day she saw the shirt was selling for 24.60. what is the percent increase

You are trying to solve what percent of the difference between $26.60 and $18 is of $24.60, so:

$24.60 - $18 = $6.60

24.60x = $6.60
x = 6.60/24.60
x = 0.2683

So the percent increase is 26.83%