When you say equivalent expression do you mean simplified expression?
If so, for this example I will write it out using ^ to represent the exponent
Writing the expression you wrote with the ^ is:
6^21 x 10^7/30^7
Using some laws of exponents
a^m x b^m = (axb)^m
a^m/b^m = (a/b)^m
and factors we can simplify the above.
Express 6^21 as (6^3)^7:
(6^3)^7 x 10^7/30^7
Perform multiplication and division of bases and raise to common power of 7
(6^3 x 10/30)^7
Express all numbers as factors to simplify base (or can multiply and divide)*
((6x6x6) x (2x5)/(6x5))^7
((6x6x6) x (2x5)/(6x5))^7 = (6x6x2)^7 = 72^7
*If you multiplied and divided would get (2160/30)^7 = 72^7