Will someone come and me with this math ?

While scuba diving, rajeev dove to a depth of 12.6 feet below the surface of the water and then descended another 8.7 feet. what expression can be used to find rajeev's new position? 12.6 – 8.7 –12.6 – 8.7 –12.6 – (–8.7) 12.6 – (–8.7)

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie

Asphere has a diameter of 10 in. what is the volume of the sphere?