How Do You Calculate Initial Acceleration in Newton's Laws Problems?

[Bensky]

[SOLVED] Need help on a Newton's Laws problem :(

Homework Statement

A 56 kg ice skater holds up a large sheet of cardboard that can catch the wind and drive her (frictionlessly) across the ice. While she's moving at 0.5 m/s, a wind that is constant and horizontal for 5.0 s exerts a force on the cardboard of 2.1 N. What is the skater's initial acceleration once the wind begins to blow?

Homework Equations

f=ma
a=change in velocity/change in time

The Attempt at a Solution

I'm not sure how to find the initial acceleration. I know how to find the average acceleration though.

 

[Bensky]

Nevermind, figured it out on my own:
Just have to do: 2.1/56

>_< why did i think this was hard...