Let's draw an illustration of the right triangle given their sides.
In order to determine the length of the third side, we will use the Pythagorean Theorem.

where "a" and "b" are the sides of the triangle that are perpendicular to each other while "c" is the hypotenuse or the third side.
Let a = 5.30 m and b = 7.80 m. Let's plug this into the formula above.

Then, solve for c.



Therefore, the length of the third side is approximately 9.43 meters.