82 is the hypotenuse.
Follow the c² - b² = a² formula, in which c is the hypotenuse.
Plug in the numbers
(82)² - (80)² = (a)²
Simplify.
82 x 82 = 6724
80 x 80 = 6400
6742 - 6400 = a²
Subtract:
a² = 6724 - 6400
a² = 342
Finally, root both sides
√a² = √342
a = √342
a = 18.49
You're closest answer through rounding is (A) 18.
hope this helps