Given a 10 ft length tree trunk and with a radius of 3 ft what is the weig of the tree trunk section if its density is 45lb/ft*3

The tree trunk is a cylinder so we apply that formula:

It is VCylinder = density x pi x radius x height
So the given in our problem is:
Density = 45 lb/ft^3 = 720.8 kg / m³
Radius = 3ft 
Height = 10 ft

Insert variables that in our equation:

VCylinder = 720.8 kg / m³ * pi * 3ft² * 10 ft =  5771.26 kgs

The variables should add up to equal 1