Answer:
21.9
Explanation:
The altitude of an isosceles triangle bisects the base. So, x represents the hypotenuse of a right triangle with legs of 9 and 20. It can be found using the Pythagorean theorem:
x^2 = 9^2 +20^2 = 81 +400
x = √481 ≈ 21.932
The length x is about 21.9 units.