A $40\mathrm{.}0$ kg dolphin decelerates from $12\mathrm{.}0$ to $6\mathrm{.}00$ m$/$s in $3\mathrm{.}00$ s to join another dolphin in play. What average force $($in N$)$ was exerted to slow him if he was moving horizontally? $($The gravitational force is balanced by the buoyant force of the water. Assume the positive direction is in the direction of motion. Indicate the direction with the sign of your answer.$)$