a) The mass of the dolphin can be calculated by using Newton's second law:

where
F=320 N is the force experienced by the dolphin
m is the dolphin mass
is the acceleration of the dolphin
Rearranging the equation and substituting the numbers, we find the mass of the dolphin:

b) If the mass of the dolphin is halved, the new mass is:

And by using again Newton's second law, we can find the new acceleration of the dolphin:

so, the acceleration is twice the original acceleration.