3(repeat s1 and switch the first two letters)=> CBA
4(repeat s2 and switch the last two letters)=> CAB
So the original order ABC reappears at step6(D)
Arc SBT equals (2*6*π(pi))/4=3*π(pi)
Because ARCB is a rectangle, its diagonals are equal, so AC=RB=6(Radius)
The perimeter of the shaded region equals Arc SBT+(SA+CT)+AC=3*π(pi)+4+6=10+3*π(B)
Hope that helps.