Why Is My Calculation of Acceleration Marked Wrong?

[oooo]

Here is a problem from our freshman physics homework--it seemed pretty simple and self-explanatory:
(refering to a diagram) 22.5 Newtons of force must be used to pull the 4 kg block across the rough surface at a constant velocity of 6.7 m/sec to the right. If the string pulling the block rightward were to break, what would be...
(a) the acceleration of the block:
Now, isn't the formula for acceleration a=F/m?
Wouldn't a = 22.5N/4kg = 5.625 m/s^2? It seems simple enough, yet it is marked wrong. If I made a silly mistake, I would like to know why, because it seems really easy and I don't know where I went wrong.
THEN, after that, it prompts me to find the time it would take for the block to stop. Not quite sure how to figure that out... any suggestions?

 

[Hootenanny]

Wouldn't a = 22.5N/4kg = 5.625 m/s^2?

Nope. What is the direction of the force acting relative to the velocity of the block?
HINT: You are very close.

~H

 

[oooo]

ooh, so it is -5.625! Thanks!
OH! and about my time question, I thought about it some more and I figured it out (no need to help) but thanks though!

 

[Hootenanny]

ooh, so it is -5.625! Thanks!

My pleasure.

~H