Here sender and receiver agree to use a modulus n=23 and g=5.
Sender secret key=6
Receiver secret key=15.
Shared key that is used by both sender and receiver in diffiehelman key=$g^{ab}mod p$=$g^{ba}mod p$.
=$5^{15*6}mod 23$
=$5^{90}mod 23$
=$[5^{20}mod 23][5^{20}mod 23][5^{20}mod 23][5^{20}mod 23][5^{10}mod 23]$
First solve $[5^{20}mod 23]$=$[5^{5}mod 23][5^{5}mod 23][5^{5}mod 23][5^{5}mod 23]$
=(20*20*20*20)mod 23=12.
=(12*12*12*12*9)mod 23
=2.
Hence secret key is=2.