find the value of a, for which point p(a/3,2) is the midpoint of the line segment joining the points Q(-5,4) and R(-1,0)

Me its important ! marge runs an ice cream parlor. her speciality is triple chocolate sundaes.she can prepare 1 sundae every 2 minutes, and she earns $1.20 for each sundae she makes . if she just makes sundaes for a single shift of at most 4 hours and at least 2 hours , which function relates her earnings to the number of minutes she works?