Answer:
increased by 9 times
Step-by-step explanation:
As the period of a simple pendulum is defined as the following formula
![T = 2\pi\sqrt{(L)/(g)}](https://img.qammunity.org/2021/formulas/physics/college/d2ynzlwad2imfwu2xmdnt0rsy4ysm6aqmo.png)
where L is the length of the pendulum and g is the constant gravitational acceleration. Since L is under a square root, if you want to triple the period of vibration, L must be increased by 9 times so that
![√(9) = 3](https://img.qammunity.org/2021/formulas/physics/high-school/eg73b6oifndnhw69b4q3fq9kqbaoy8npdb.png)