Its simple just follow the formula (x^a) / (x^b) = x^(a-b).
See now x^3 / x^7 becomes x^(3-7) = x^-4 it can also be written as 1/(x^4).
Now apply for y.
y^7 / y^2 = y^(7-2) = y^5
Now by combining the two fractions, x^3 y^7/ x^7 y^2 = y^5 / x^4.
Hope it helps.
