There are nine water bottles in​ Devin's refrigerator. He adds three full boxes of water bottles to the refrigerator. Then he adds two more boxes that each has 1 fewer bottle than a full box. When he is​ done, there are 67 bottles in the refrigerator. Write and solve an equation to find the number of bottles in a full box.

(Type an equation using b as the variable. Do not​ simplify.)