What is the resistance of a 20-cm-long column of blood in this artery? assume that resistivity of blood is equal to 1.6 ωm.

The way in which you need to solve this is by using the formula:
R = ρL/A 
That one is the formula that demonstrates Resistance of a wire in Ω.
The formula can be explained like this: 
ρ is resistivity of the material in Ω-m 
L is length in meters 
A is cross-sectional area in m² 
A = πr², r is radius of wire in m 
As soon as you have your numbers, you can replace and proceed to get the asnwer

When you are making a guess you are forming a hypothesis