How to create formula to sum with dynamic ranges? I just can't make the right syntax

So what I would like to do is add a sum formula to a cell, so it can be edited later(normally, not through vba). It sums up some cells, but the amount of cells is not always the same. Sometimes it's 4 cells, sometimes it's 10. So I'm trying to have:

lastrow = Sheets(7).Cells(Sheets(7).Rows.Count, "A").End(xlUp).Row

then using it:

Sheets(7).Range("B" & lastrow + 1).Formula = "=SUM(b2:b&"lastrow")"

My problem is the syntax actually, I can't seem to make it right. How to add lastrow to this formula?

Hope it's understandable, English is not my native language.