Afootball player with a mass of 85 kg wears a uniform and helmet that have a mass of 4.5 kg. the football player moves at 2.1 m/s. what is the total momentum of the football player and uniform?

The answer would be 187.95 kg.m/s.

To get the momentum, all you have to do is multiply the mass of the moving object by the velocity. 

p = mv

Where:
P = momentum
m = mass
v = velocity

Not the question is asking what is the total momentum of the football player and uniform. So we need to first get the combined mass of the football player and the uniform. 

Mass of football player = 85.0 kg
Mass of the uniform     =   4.5 kg
TOTAL MASS                  89.5 kg

So now we have the mass. So let us get the momentum of the combined masses. 

p = mv
   = (89.5kg)(2.1m/s)
   = 187.95 kg.m/s

Ibelieve the answer is d. densityhope my answer has you : )