Answer:
72 units
Explanation:
The Pythagorean Theorem states that
in a right triangle when a and b are the two legs and c is the hypotenuse (the longest side).
In the diagram, 30 measures one of the legs and 78 measures the hypotenuse. Plug these in as a and c.

Subtract 900 from both sides

Take the square root of both sides

Therefore, the length of the missing side is 72 units.
I hope this helps!