A 0.40kg piece of putty is dropped from a height of 2.9m above a flat surface. When it hits the surface, the putty comes to rest in 0.32s .

What is the magnitude of the average force exerted on the putty by the surface?

I keep getting 9.4 as the answer, but it says it's wrong. I first calculated the velocity using sqrt(2gh) and got 7.539 m/s. And then solved for force using mv/t. Please tell me what I'm doing wrong.