Answer:
21x,
where x = number of entrees
Step-by-step explanation:
appetizer choices: 7
choices for an entree: x
choice for dessert: 3
Total number of combinations:
7 * x * 3 = 21x
Since the number of choices of entrees is missing, the best answer you can get is 21x, where x is the number of entrees.
Replace x with the number of entrees to get your final answer.