Need some help (Geo)

Another type of subtraction equation is 16-b=7. explain how you would sole this equation then solve it.

Identify all the cases where non-probability sampling is used when attempting to study the characteristics of a large city. studies on every subject in the population divide a population in groups and obtain a sample studies with a limited amount of time demonstrate existing traits in a population studies with the nth person selected

Which is the value of this expression when p=3 and q=-9