Please assist with this question

Number of Marginal Cost from Average Cost=Total Patients per Year Total COSt previous output rate Cost/ Number of Patit 0 $1000 ----- 1 $4500 $3500 $4500 2 $7500 $3000 $3750 3 $10000 $2500 $3333 4 $12000 $2000 $3000 5 $14500 $2500 $2900 6 $17500 $3000 $2917 7 $21000 $3500 $3000 8 $25000 $4000 $3125 9 $30000 $5000 $3333 What is the number of patients that gives Joan's the most prot, if the price patients pay is $4200