Answer:
 4 bottles
Step-by-step explanation:
Let o represent the number orange juice bottles Megan bought. Then the total number of juice bottles is ...
 o + 12 = 4o
 12 = 3o . . . . . . . subtract o
 12/3 = o = 4 . . . . divide by the coefficient of o
Megan bought 4 bottles of orange juice.