Cliford, Arena is a done deal. You can easily calculate the amount of shares you got :

"...Pursuant to the Arrangement Agreement, Arena’s shareholders (“Arena Shareholders”) will receive 0.0226 (the “Exchange Ratio”) of a Lithium Americas common share (a “LAC Share”) for each Arena Share held (the "Consideration"). The Consideration to Arena implies a total equity transaction value (on a 100% basis) of US$227 million (C$311 million), based on the closing price on December 19, 2022 ..."

If you have 10000 shares of Arena for example:

so 10,000 x 0.0226 = 226 shares of LAC

On Dec 19, LAC was $21.61 - that's how your investment will look no matter what happens to LAC.

Arena SP is following approximately LAC SP from now on staying close to that conversion factor. I personally sold my Arena and converted to LAC, versa waiting a year for them to do it. You might get bit fewer shares but it is free for you to do whatever you want. As always it is an individual decision.