Stelios,
Here is one solution:
Break down your equation so each part is in it own column.
Column A   Column B Column C     etc.
   (                 2            +             etc.

Somewhere on the same worksheet list the numbers(0 -9) and the
operators (*, /, +, - ). I used R3:R12 for numbers and S3:S6 for
operators.
Column A   Column E  Column G   Column K
    (                )              (                )
Column B, C, D, F, M
Use Validation to do a drop down "List".

Column L uses an "IF statement"
"=IF(C2="*",(B2*D2),IF(C2="/",(B2/D2),IF(C2="+",(B2+D2),_
IF(C2="-",(B2-D2),0))))"  Leave off the first " and last ". The "
inside the IF statement are necessary.

Column N uses an "IF statement"
"=IF(I2="*",(H2*J2),IF(I2="/",(H2/J2),IF(I2="+",(H2+J2),_
IF(I2="-",(H2-J2),0))))"  Leave off the first " and last ". The "
inside the IF statement are necessary.

Column P uses an "IF statement"
"=IF(M2="*",(L2*N2),IF(M2="/",(L2/L2),IF(M2="+",(L2+N2),_
IF(M2="-",(L2-N2),0))))"   Leave off the first " and last ". The "
inside the IF statement are necessary.

You can copy this down, and change the numbers and the operators
and the answer in column P will change with the ne information.

hth

This will do it.  Let me know what grade we got.

Sub Main()
 Dim ch$, strOld$, strNew$, lenTmp%
 Dim iPos%, iPosLParen%, iPosRParen%, iCol%
 iCol = 2
 strOld = Cells(1, 1).Value
 Do
   strNew = ""
   iPosRParen = 0
   For iPos = 1 To Len(strOld)
     ch = Mid$(strOld, iPos, 1)
     strNew = strNew & ch
     If ch = "(" Then
       iPosLParen = iPos
     ElseIf ch = ")" Then
       iPosRParen = iPos
       If iPosLParen <> 0 Then
         lenTmp = iPosRParen - iPosLParen + 1
         strNew = Left$(strNew, Len(strNew) - lenTmp) & _
         Application.Evaluate( _
           Mid$(strOld, iPosLParen + 1, lenTmp - 2))
       End If
       iPosLParen = 0
     End If
   Next iPos
   If iPosRParen = 0 Then strNew = Application.Evaluate(strNew)
   Cells(1, iCol) = "'" & strNew
   iCol = iCol + 1
   strOld = strNew
 Loop While iPosRParen <> 0
End Sub ' Dave D-C