Dario was scuba diving at an elevation of −20 feet and then descended 20 feet more. Dario and another person scuba diving. The other person is diving at negative thirty feet.

A number line from negative fifty to fifty by tens is shown.

After the descent, what was his elevation in relation to another diver at an elevation of −30 feet?

They start at -20 and descend so subtract the amount they descend from the starting value:

-20 -29 = -40

Dario is now at -40 feet.

The other diver is at -30 feet so Dario is 10 feet below Dario.

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.