Powershell rename computer error missing ')' method call.

I have this powershell command to rename a computer name based on the 2 variables I set. But when I try to execute, it throws an error,  missing ')' in method call.

(Get-WmiObject win32_computersystem).rename($n1$n2).  I tried ('newname') and it works, but wont work with variable. btw, this is a domain joined computer.