(a) about how many microorganisms are found in the human intestinal tract? (a typical bacterial length scale is one micron = 10−6 m. assume the intestinal volume is 7 m with a diameter of 4 cm and assume bacteria occupy one hundredth of it.)

First, you need to estimate the volume of the human intestinal tract. To estimate this, we can use this:  
 
pi r^2 h = 3.14 x 0.02^2x7 = 0.009 m^3 =9 x 10^-3 or almost 10^-2 m^3  
 
The volume of a bacterium is approximately 10^-18 m^3 (this is the volumes being cubes of linear dimensions)  
 
if the intestinal tract is 1/10 full, then there are 10^-3 meters available for filling, and there are:  
 
10^-3 m^2/10^-18 m^3/bacterium = 10^15 bacteria