PNK goes to PENN and I lose money!

Nov 10, 2018
On 10/15/2018 I was notified that my 20 shares of Pinnacle Entertainment (PNK) worth $32.55 per share were being traded in for 8.4 shares of Penn National Gaming worth $30.90 a share. This is due to a corporate acquisition. So I incur a loss of $391. Just like that! My online brokerage says those are the terms of the acquisition. How can they get away with this? They trade 20 shares for 8.4 shares of a cheaper stock and I can take a flying leap? Why is this allowed to happen? Is there, in fact, anything I can do about it? Anybody know anything about this?