suppose b is congruent to a^67 (mod 91) and (a,91)=1 (gcd) for some a,b element of Z such that 0<=a<91.

1) find k element of Z such that b^k is congruent to a (mod 91)

2) let b=53. Find a

i think 1 & 2 need to be done as separate questions but I have NO clue how to do this.

any help is appreciated.