A gymnast jumps straight up, with her center of mass moving at 2.80 m/s as she leaves the ground...?

How high above this point is her center of mass at .100 seconds? I tried using the formula Xf=xi+Vit+1/2at^2, but ended up with .329. The answer in the back of the book says .231. I just need to know how to solve the problem. Thank you. (Freely falling objects neglect air resistance)