Question

1/7xy(7xy)^3

Answer 1

1/(7xy)^4=1/2401(xy)^4

Answer 2

i see nothing wrong with this answer
some people might read your question differently then what jamal read

Answer 3

i assume you mean
\[\frac{1}{7xy} \cdot (7xy)^3=\frac{1}{7xy} \cdot 7^3x^3y^3=7^{3-1}x^{3-1}y^{3-1} \]
but honestly your problem isn't clear enough
other interpretations could be made