The process is:
C = 2nr
2nr = c
r (2n) = c
That's the answer☝️☝️☝️☝️
or maybe this one
r = 2n + c
sorry, I'm unsure, it has been too long since I studied that...
CD=CA+AB+BD
AB=CD-CA-BD
AB=CD-BD-BD
AB=CD-2.BD
AB=CD-2(BC+CD)
AB=CD-2BC-2CD
AB=-(CD+2BC)
AB=-{98-6(x+8)}
AB= 6x-50