Cluster sampling and stratified sampling both involve selecting subjects in subgroups of the population. what is the difference between those two types of sampling?

In clustering sampling, the researcher test whole groups of people. In stratified, it's systematic choosing.

A census could be: which literary character is more popular in America: Harry potter or Sam Gamgee (Lord of the Rings)?  

First you would choose states to do the testing in, then cities. some of the clusters of people you choose will be tested as a whole (cluster testing). then, you would take random people from the other groups which were not tested as a whole to answer the question. (stratified sampling)

For both methods, divide a population into groups.

For cluster sampling, select all of the members in some of the groups.

For stratified sampling, select a random sample from each group.

you can't solve x because it is 2 linear equation and you only have one equation