Can someone help me with the questions in the picture?

Which equation can be used to measure the density of water in a spherical water tank? let w represent the number of water particles within the region.

Hey everyone! this question is a challenge question. for 50 points if you wish. all you have to do is answer the correct it has to be correct, in under 50 seconds, and i will make you the ! you want to accept this challenge. i already know! i mean, it's 50 free points! here is the question: 11x2= answer this correct, you get two prizes. don't forget. you only have 50 seconds or it wont happen. get it correct in under 50 seconds! that's not bad. so stop reading this long thing and !

What is the 8th term of the geometric sequence with this explicit formula? an= )(n-1)