Answer:
22.5 weeks
Step-by-step explanation:
y = 1.12x + 28.78 represents the height of the puppy.  Here that height is 54 cm:
Then 54 = y = 1.12x + 28.78
Combining the constants, we get 1.12x = 25.22.  Dividing both sides by 1.12, x turns out to be 22.5 weeks.
 
        
             
        
        
        
I've attached a plot of one such cross-section (orange) over the region in the x-y plane (blue), including the bounding curves (red). (I've set 

 for this example.)
The length of each cross section (the side lying in the base) has length determined by the horizontal distance 

 between the y-axis 

 and the curve 

. In terms of 

, this distance is 

. The height of each cross section is twice the value of 

, so the area of each rectangular cross section should be 

.
This means the volume would be given by the integral
 
 
        
        
        
Answer:
Tonya cut the cylinder vertically and David cut it diagonally.