Open network folder

I got the folder open when I ran Start '\\PCName\c$\Users'

But I got error when I ran

$a = 'PCName'

Start '\\' + $a + '\c$\Users'

The error is

Start-Process : A positional parameter cannot be found that accepts argument 'PCName'.
At line:1 char:1
+ Start '\\' + $a + '\c$\Users'
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidArgument: (:) [Start-Process], ParameterBindingException
    + FullyQualifiedErrorId : PositionalParameterNotFound,Microsoft.PowerShell.Commands.StartProcessCommand