If we want to calculate 12xy^3/4xy^2, we divide the constants (12/4) and arrive 3, but then we simply subtract the power of the variable in the denominator from the power of the variable in the numerator. Here for x, that is 1 - 1, leaving us with 0. Thus the value of a is 0, and the x^0 term would just become a 1, allowing us to essentially remove it since it simply is multiplying by 1.