I honestly don’t know the answer; I’m quite confused.
Response:
The average depth is 12.75 cm.
The depth variance is calculated as 13.02 cm².
Clarification:
Uniform probability occurs when every outcome is equally probable.
In this scenario, we have a lower limit of the distribution denoted as a and an upper limit referred to as b.
The mean of the uniform distribution is:

The variance of the uniform distribution is represented as:

Uniform distribution across the interval (6.5, 19)
Thus, we have: 
So
Mean:

Average depth is 12.75cm.
Variance:

The variance of depth equals 13.02 cm².