Perhaps you meant [(√(x^7))^(1/5)]^3
The fifth root of the square root of x to the seventh cubed?
√(x^7)=(x^7)^(1/2)=x^(7/2)
(x^(7/2))^(1/5)=x^(7/10)
(x^(7/10))^3
x^(21/10)
Sorry I did my best to decipher what you wanted to express.
In general:
(a^b)^c=a^(b*c) just work from the inside to the outside...
