you know that you need to wait for server 1 to be done with the guy, before you can go to server 1.
once you are at server 1, if you finish earlier, u have to wait for server 2.
if you finish later, u go immediately to server 2
lastly, you will have to wait to be done with server 2.
E(time of you being done) = E(server1 serving guy)+ E(Max(server2 serving guy, server1 serving you)) + E(Server2 serving you)
= 1/mu_1 + (1/mu_1 + 1/mu_2 - 1/(mu_1+mu_2)) + 1/mu_2