for problems like this you can move the y's on the bottem up top by fliping the sign of the exponent (in this case 7 to -7) and MULTIPLYING it with EVERYTHING on top, then because of properties of multplication you can add the exponents to combine them into one term (in this case add 3 and -7 to get -4)
[tex] \frac{ y^{3} }{y^{7}}=y^3 *y^{-7} = y^{3+-7}=y^{3-7}=y^-4[/tex]
If any of this does not make sense let me know and ill try to clarify better
