Final answer:
The mass of the box is 9 kg.
Step-by-step explanation:
To find the mass of the box, we can use Newton's second law of motion, which states that F = ma, where F is the force applied, m is the mass of the object, and a is the acceleration. Rearranging the equation, we have m = F/a. In this case, the force applied is 99 N and the acceleration is 11 m/s². So, the mass of the box is 99 N / 11 m/s² = 9 kg.