Brandon wants to ride his bicycle 21 miles this week. He has already ridden 6 miles. If he rides for 5 more days, write and solve an equation which can be used to determine xx, the average number of miles he would have to ride each day to meet his goal.

Equation: 5x+6=21

Solve: 5x+6=21
-6= -6
5x=15
5x/5 = 15/5
x =3
Answer: x = 3