Multiplying two fractions together corresponds to multiplying together their components, so multiplying -9/10 · -2/3 looks like:
If we rearrange the fractions, we can perform some simplification:
When gets us:
This is an upwards-opening parabola with a vertex of (2, 0). That +2 moves the vertex up 2 along the y axis. If this is a positive parabola, which it is, then the vertex is a minimum.
I believe the submarines final location is 126 feet below sea level because if you multiply 25 1/5 by 5 you get 126.