One side of a seesaw carries a 21kg mass four meters from the fulcrum and a 25.5kg mass two meters from the fulcrum. To balance the seesaw, what mass should be placed nine meters from the fulcrum on the side opposite the first two masses?
a. 36 kg
b. 18 kg
c. 45 kg
d. 12 kg
e. 15 kg​