Answer:
9 units
Concept Used:
Pythagorean Theorem: a²+b²=c²
(a: Perpendicular, b: Base and c: Hypotenuse of the right-angled triangle)
Surds Operations
Explanation:
It is evident that the Hypotenuse is the missing side.
Using Pythagorean Theorem:

c = +9 units (distance is a scalar quantity and cannot be -ve)