Discrete Mathematics - Modular arithmethics I've been trying to solve a task for some while now but I'm currently very stuck.

For the integers $a$ and $b$ it applies that $b \equiv a \pmod{91}$ and $\gcd (a, 91) = 1$. Determine a positive number $k> 1$ such that $b^k \equiv a \pmod{91}.$

I know that  $b^{\phi(91)} = 1\pmod{91}$ and  $b^{\phi(91)} = 72$.
After this I'm pretty empty. Would anyone be so kind to guide me or help me in the right direction?

$k$ calculate as discrete logarithm by modulo $91$ and have form $k=r\pmod{m}$, where $r$=2..11 and $m$=3,4,6,12.
gp-code:
k91()=
{
 K= [];
 for(a=0, 90, if(gcd(a,91)==1, for(b=0, 90, if(gcd(b,91)==1,
  m= Mod(b, 91);
  h= znorder(m);
  k= znlog(a, m, h);
  if(k, if(k>1,
   K= concat(K, [[k,h]]);
  ))
 ))));
 print(vecsort(K,,8))
};

Output [r,m]:
[[2, 3], [2, 4], [2, 6], [2, 12], [3, 4], [3, 6], [3, 12], [4, 6], [4, 12], [5, 6], [5, 12], [6, 12], [7, 12], [8, 12], [9, 12], [10, 12], [11, 12]]

