First, write a statement to define your unknown variable. This is technically not essential to solving the problem, but many teachers prefer to see it.
Example: Let w = the manatee's weight in pounds.
Next, decide what the variable needs to be multiplied by. 12% is equal to 0.12. Usually when a problem says "of," it wants you to multiply. Therefore, 12% of the body weight equals 0.12w. Since the problem already says that this is how much the manatee eats every day, that is your answer.
r^2*h/3