Answer:
 Mark earns in a day is y = 18h + 0.05s .
Explanation:
As given
Mark works at a popular electronics retail store. 
He earns a fixed income of $18 an hour and a 5% commission on the sales he makes.
As given
 y = earnings .
 h = number of hours worked 
s = amount of sales 
5% is written in the decimal form.

= 0.05
Mark earns through commission = 0.05 × s
 = 0.05s
Mark earns h working hours = 18h
Thus
Total earning of the mark = Mark earns h working hours + Mark earns through commission .
Put the value
y = 18h + 0.05s 
Therefore the Mark earns in a day is y = 18h + 0.05s .