Answer:
x - 6m
Explanation:
Let the amount of gift certificate be x
If $6 is deducted everytime she watches as movie, then the total deduction for m movies watched will be $6m.
Her gift balance certificate = initial gift balance she has - total deduction for m movies
Her gift balance certificate = x - 6m
Hence the expression that can be used to determine her gift balance certificate is x - 6m