New Finite Geometric Series Sum exercise

I have created an exercise for this video: https://www.khanacademy.org/math/trigonometry/seq_induction/advanced-sequences-series/v/formula-for-finite-geometric-series
It is my first attempt to create an exercise, so I will be grateful for any feedback, advice or error notifications. And I would also like to ask for corrections of my English in the exercise - I am not a native English speaker. (Sorry for this, I have done my best, but English is not my strong point.)