Lisa and Daisy work at a hair salon. The salon charges $21 for a hair styling session with Lisa and $17 for a session with Daisy.

Their income on a certain day is projected to be $357. This situation can be represented by the equation 21x + 17y = 357, where x is the number of Lisa's customers and y is the number of Daisy's customers.

(Technical details: x ≥ 0, y ≥ 0, and x and y take only integer values.)

How many customers would Lisa need to serve to attain the projected income if Daisy calls in sick that day?