__+ 13= -8 what is the unknown addend

Technicians have installed a center pivot, circular irrigator for a farm that covers 25 acres. a.) how long is the irrigation boom (in feet) from the center pivot to its farthest extent? b.) if the irrigation boom makes 4 full revolutions in a day, how many miles does the end of the boom travel in 5 days?

What is the answer to this problem? ignore the work. what is the correct answer?

What is the exact volume of the cone? (height is 9 cm, the radius is 15 cm)