Identify what information is given and what is asked for:
mass of the ball
Velocity of the ball
mass of the ball must be converted to S.I. units 420g = 0.42kg
$p=m \times v$
$=0.42 \times 20$
$=8.4$ kg.m/s in the direction of the goal post, because your answer is positive. If your answer had been negative the direction would be away from the goal post.