Subtracting Monomials
    To subtract a monomial from another of the same variables just subtract the coefficients. Only two like monomials can be subtracted from each other.
    Example:
    By subtracting 3x from 4x, we get (4-3)x=x.
    By subtracting 3xy from 2xy, we get (2-3)xy=-xy.
    By subtracting bx from ax, we get ax-bx=(a-b)x.
    Remember:
    Monomials can be subtracted only if they have the same variables with identical powers.
    Combinations of Choices
    If there are n choices for one type and m choices for second type, then number of combinations for both types is the product of choices:n*m.
    Example:
    If there are 3 kinds of breads to choose from and 4 kinds of meats, then we can make 3*4=12 different sandwiches.
    If an alphabet has 26 letters, then the number of possible 2 letter word is 26*26=676.All of them may not be vaild words.
    Remember:
    Do not add the choices to get the total combinations.
    The product rule can be applied to more than two types of choices: if there are 3 kinds of breads, 4 kinds of meats, and 6 kinds of cheeses, then we can make 3*4*6=72 different sandwiches.