Incomplete Momentum bar chart for collision

[chicagobears34]

Homework Statement

The figure (Figure 1) is an incomplete momentum bar chart for a collision that lasts 15ms .

Figure 1: http://session.masteringphysics.com/problemAsset/1384208/4/knight_Figure_09_06.jpg

What is the magnitude of the average collision force exerted on the object?
Express your answer to two significant figures and include the appropriate units.

Homework Equations

Pix+Jx=Pfx

The Attempt at a Solution

I've never seen a momentum bar chart before and we never learned anything like this in class.
i tried -8N because 6-8=-2, but that was wrong
How am I supposed to approach this problem?

 

[Sunil Simha]

-8 is actually the magnitude of change in momentum. You must divide it by the time taken to get the average force.

 

[chicagobears34]

yes, that makes sense and it is correct.
Thanks Sunil!