The weight of a cylindrical can varies jointly with the height of the can and the square of the cans radius. a 4-inch high can with a 1.5 inch radius weights 16 ounces. a) find the constant of proportionality, k. b) what is the weight of a can made of the same material that is 4.5 inches high and has a 2 inch radius?

With the information given we have:

Where W is the weight, k is the proportionality constant and r is the radius of the can's radus. Remember that the weight varies jointly so this key word means that you need to multiply the mentiones parameters by a proportionality constant k. 
We simply need to reorganize the expression and isolate k for the frst

Replacing by all the appropiate values we get:

For the second part knowing k we have: