There are 78 municipalities in Puerto Rico.
Names from CIA World Factbook (http://www.cia.gov/cia/publications/factbook/geos/rq)
... on x) to the expression to the left of "=", that will make it a perfect square trinomial of the form x2 + 2xy + y2. Since "2xy" in this case is (b/a)x, we must have y ...