How can I use invoke code with powershell to sent multi email

I can send multi email by invoke code with powershell as follow :
Send-MailMessage -From ‘a@gmail.com’ -To ‘b@gmail’, ‘c@gmail’ …

but when i use variable to replace email, it can’t work as follow :
$mailFrom = “a@gmail.com
$mailTo = “b@gmail.com, c@gmail.com
Send-MailMessage -From $mailFrom -To $mailTo …

how can I use variable to replace mailTo

You can reference workflow variables as powershell variables $variablename )
See example here