I am trying to solve a cipher using f(x) = 3x+15 (mod 26). Going in order with A=0 all the way to Z=25. So the encoding for the letter A was A -> 0 -> 3*0+15 (mod 26) = 15 -> P.
so im getting A=15=P, B=18=S, C=21=V, D=24=Y...
i was trying to cipher ' PSRUOPVU '
and im getting: IROXFIAX.
Is this right???