Trying To Add Line To VBA

I am trying to add this line to the script in order for the "SaveAs" function to rename the file as the text in the quotes, and the cell valve.

Code:
"Req-" & Range.("P3")

This is what I currently have. This is not my VB, I forgot where I copied it from.

Code:
Sub Save_As()
Dim file_name As Variant

' Get the file name.
file_name = Application.GetSaveAsFilename( _
FileFilter:="Excel Files,*.xls,All Files,*.*", _
Title:="Save As File Name")

[Code] ......