Take into account that the impulse is given by:
where,
m: mas of the ball = 3.0kg
v: final speed = 3.0m/s
vo: initial speed = 9.0m/s
replace the previous values into the formula for I and simplify:
Hence, the impulse is -18 kg*m/s (it's negative due to the force is opposite to the motion of the ball)