Adding sum of a dynamic range - excel

Question

New to stack here.
I'm on VBA and am creating a quoting tool where outputs are generated if certain cells are populated from row 57 onwards. I.e. "For a = 57 to 1000".
For each of row "a" that has values, I'm trying to have Cells(a + 1, 6) output the sum of the previous a columns.
It took some time to find how to reference a range using cells, but I've been stuck from there. To reference the range I've found something along the lines of below
With Sheet1
.Range(.Cells(a, 6), .Cells(a + 1, 6)).Value2 = "b"
End With
The .Range(.Cells(a, 7), .Cells(a + 1, 7)) are the cells that I want to create a sum for but I'm not sure how to do this (and whether the With Sheet1 and End With sections are needed.
Full VBA code here:
Sub OnEntry()
On Error Resume Next
For a = 14 To 1000
If IsEmpty(Cells(a, 3)) = False And IsEmpty(Cells(a, 4)) = True Then
If IsEmpty(Cells(a + 2, 4)) = True And IsEmpty(Cells(a + 3, 4)) = True And IsEmpty(Cells(a + 2, 3)) = True Then
GoTo GoHere
End If
If IsEmpty(Cells(a, 4)) = True And IsEmpty(Cells(a + 1, 4)) = False Then
GoTo GoHere
End If
Find = Range("'Sheet1'!$B$6:$B$5000").Find(Cells(a, 3).Value).Address
Row# = Range(Find).Row
Cells(a, 4) = Sheets("Sheet1").Cells(Row#, 3)
Cells(a, 5).Value = "=VLOOKUP(C" & a & ", 'Sheet1'!$B$6:$F$5000, 4, 0)"
Cells(a, 7).Value = "=B" & a & "*E" & a
If Cells(a - 1, 1) = "Line" Then
Cells(a, 1).Value = 1
Else
Cells(a, 1).Value = Cells(a - 1, 1) + 1
End If
Cells(a + 1, 1).EntireRow.Insert
**Stuck here**
With Sheet2
.Range(.Cells(a + 1, 7), .Cells(a + 2, 7)).Value2 = "b"
End With
End If
Next a
GoHere:
WaitTime = Now() + TimeValue("00:00:02")
While Now() < WaitTime
DoEvents
Wend
End Sub