HELP!

Hint:   add all the digits of 42087591834593475

If that result is divisible by 3, then 3 is the GCF of both numbers.......if the sum is not divisible by 3, then both numbers are relatively prime  [ have no factors in common]

Since 3 only has two factors 1 and 3, you can only hope 42087591834593475  is divisible by 3

42087591834593475 /3 = 14029197278197825

And you can see it is