Find the lowest common denominator of 3/x3y and 7/xy4

The lowest common denominator is : 
xy 
first is 3/x²/xy 
the second 7/y²y/xy

The lowest common denomenator is the least common multipule of the denomenators

denomenator 1 is x^3y
denomenator 2 is xy^4

factor them
denom1=x*x*x*y
denom2=x*y*y*y*y

so the combination of both is
x*x*x*y*y*y*y or x³y⁴

the least common denom is x³y⁴

21, i'm pretty sure it's right lol