Answer:
![h = 67.5\,m](https://img.qammunity.org/2020/formulas/physics/college/a4bd7pge5fej8o2f2gk54tw8w4qd2jgx2c.png)
Step-by-step explanation:
The position of the cannonball is given by the following expressions:
Half position
![h = H -(1)/(2)\cdot g \cdot \left((t_(g))/(2) \right)^(2)](https://img.qammunity.org/2020/formulas/physics/college/4le8r1m6y8472emn7sevy115tjy2pa7cb0.png)
Final position
![0 = H -(1)/(2)\cdot g \cdot t_(g)^(2)](https://img.qammunity.org/2020/formulas/physics/college/8uvmrqmx9db6rvyzjphbzlkvho1nsr0g41.png)
The instant when the cannonball hits the ground is:
![t_(g) = \sqrt{(2\cdot H)/(g) }](https://img.qammunity.org/2020/formulas/physics/college/lm5gqrr6agvvljnn13jofwa4b60yhcggp4.png)
Lastly, this result is applied in the other equation, which simplified afterwards:
![h = H -(1)/(8)\cdot g \cdot \left((2\cdot H)/(g) \right)](https://img.qammunity.org/2020/formulas/physics/college/5qabbcy0s2zb2f119kjqlr25bwq1vcvx36.png)
![h = H -(1)/(4)\cdot H](https://img.qammunity.org/2020/formulas/physics/college/2m8mmtmf0835xnipry7328z71nz0rqwei0.png)
![h = (3)/(4)H](https://img.qammunity.org/2020/formulas/physics/college/9l6ovqp64040lllmgclan4ffi4yxmcrevq.png)
![h = 67.5\,m](https://img.qammunity.org/2020/formulas/physics/college/a4bd7pge5fej8o2f2gk54tw8w4qd2jgx2c.png)