What is the magnitude of acceleration given $v_i$, $v_f$, and $x$?

Question

I'm really struggling with this question that is part of my physics assignment. Any help would be appreciated.

This is the question: A speed skater moving to the left across frictionless ice at 8.4 m/s hits a 4.5-m-wide patch of rough ice. She slows steadily, then continues on at 5.7 m/s. What is the magnitude of her acceleration on the rough ice?

Attempted answer: I used the formula a=(Vf^2-Vi^2)/2x

a=[(5.7^2) - (8.4^2)]/2(4.5) a=(32.49-70.56)/9 a=-38.07/9 a=-4.23 m/s^2

Masteringphysics tells me the answer is wrong and to "check my signs"

Answer 1

If you included the negative sign in your answer, then you aren't (quite) answering the question that was asked. Magnitudes are positive by definition; the negative sign on an acceleration (or velocity) is simply there to indicate direction.