The problem says: You slap a hockey puck towards the goal 20 m away. If the puck's initial speed is 51 m/s and it take 0.4 s to reach the goal, what is the magnitude of its acceleration?

Assuming that there is a constant acceleration on the hockey puck, you can use the formula:

s = ut + \frac12 at^2

s is the displacement, here 20 m,
u the initial speed,
t the time it takes
a is the acceleration for you to find.

Since you are asked for the magnitude, take the absolute (positive/modulus) of the value you get if it is negative.