ANSWER
-30 feet every minute.
EXPLANATION
The scuba diver's position in relation to the surface of the water changes by -90 ft in 3 minutes.
We want to find his average change in position every minute.
To do thus, we have that every 3 minutes, the position changes by -90 feet:
3 minutes = -90 feet
1 minute = x
Cross multiply:
x * 3 = 1 * -90
=> x = -90 / 3
x = -30 feet
So, every minute, the scuba diver's position changes by -30 feet
