Error on using quotation marks

[ExcelPupper]

I am trying to put sum formula for last rows of columns L, M, N however I get error when I try to run the code below and I think it's caused by the placing of quotation marks?

Sub TotalAmt()

Dim lr as String

lr = Sheets(6).Cells(Rows.Count, "D").End(xlUp).Row
Sheets(6).Range("L" & lr : "N" & lr).Formula = "=SUM(L8:L" & lr -1 & ")"

End Sub

 

[Yongle]

Sub TotalAmt()

Dim lr as Long
lr = Sheets(6).Cells(Rows.Count, "D").End(xlUp).Row
Sheets(6).Range("L" & lr & ":N" & lr).Formula = "=SUM(L8:L" & lr - 1 & ")"
End Sub

 

[Yongle]

Note that I also amended variable type to LONG

 

[ExcelPupper]

Sub TotalAmt()

Dim lr as Long
lr = Sheets(6).Cells(Rows.Count, "D").End(xlUp).Row
Sheets(6).Range("L" & lr & ":N" & lr).Formula = "=SUM(L8:L" & lr - 1 & ")"
End Sub

Thanks! Do you have any reference where I could study the proper usage of "" on coding?
Most of the time those "" are the reason for my error when running the code ?

 

[Yongle]

I simply use this method and look to see what is written to the immediate window - which is displayed in VBA editor with {CTRL} g
With complicated strings, I build up the string in stages
I only put the string in the code after I get the syntax correct

Dim lr as Long
lr = Sheets(6).Cells(Rows.Count, "D").End(xlUp).Row
Debug.Print "L" & lr & ":N" & lr
Debug.Print "=SUM(L8:L" & lr - 1 & ")"