Cook-a-Code Online Qualifier question 2

[dhruv_is_back]

hello
I am not able to understand question language so plz anyone help me so that i am getting an idea how to do that thankyou…

question link - https://www.codechef.com/CCOQ2019/problems/PERMPAIN

[vipin1407]

Prove that $\gcd(a^n - 1, a^m - 1) = a^{\gcd(n, m)} - 1$

[dhruv_is_back]

Thankyou…