To calculate the density of an object you use the density formula which is:
D=m/v
now we plug in what we're given.
m=25.0g
v=49.10mL-45.50mL=3.6mL
D=25.0g/3.6mL=6.9 g/mL
The density of the iron is 6.9 g/mL. (btw, I left the answer in significant figures)