inverse bilinear interpolation

[romana]

I have a code for bilinear interpolation in VBA, but now I need an inverse bilinear interpolation - I know the z point and y point and I need to get x point. I am brand new to VBA and have no idea how to do this. Could you tell me how to change my code to do it? Thank you

Public Function GetBilinearInterpolation(xRange As Range, yRange As Range, zRange As Range, xcoord As Double, ycoord As Double) As Double
  
  Dim xAxis() As Variant
  Dim yAxis() As Variant
  Dim zSurface() As Variant
  
  xAxis = xRange
  yAxis = Application.Transpose(yRange)
  zSurface = zRange
  
  'first find 4 neighbouring points
  
  nx = UBound(xAxis, 1)
  ny = UBound(yAxis, 1)
  Dim lx As Single 'index of x coordinate of adjacent grid point to left of P
  Dim ux As Single 'index of x coordinate of adjacent grid point to right of P
  
  GetNeigbourIndices xAxis, xcoord, lx, ux
  
  Dim ly As Single  'index of y coordinate of adjacent grid point below P
  Dim uy As Single  'index of y coordinate of adjacent grid point above P
  
  GetNeigbourIndices yAxis, ycoord, ly, uy
  
  fQ11 = zSurface(lx, ly)
  fQ21 = zSurface(ux, ly)
  fQ12 = zSurface(lx, uy)
  fQ22 = zSurface(ux, uy)
  
  'if point exactly found on a node do not interpolate
  If ((lx = ux) And (ly = uy)) Then
    GetBilinearInterpolation = fQ11
    Exit Function
  End If
  
  
  x = xcoord
  y = ycoord
  
  
  x1 = xAxis(lx, 1)
  x2 = xAxis(ux, 1)
  y1 = yAxis(ly, 1)
  y2 = yAxis(uy, 1)
  
  'if xcoord lies exactly on an xAxis node do linear interpolation
  If (lx = ux) Then
    GetBilinearInterpolation = fQ11 + (fQ12 - fQ11) * (y - y1) / (y2 - y1)
    Exit Function
  End If
  
  
  
  If (ly = uy) Then
    GetBilinearInterpolation = fQ11 + (fQ22 - fQ11) * (x - x1) / (x2 - x1)
    Exit Function
  End If
  
  fxy = fQ11 * (x2 - x) * (y2 - y)
  fxy = fxy + fQ21 * (x - x1) * (y2 - y)
  fxy = fxy + fQ12 * (x2 - x) * (y - y1)
  fxy = fxy + fQ22 * (x - x1) * (y - y1)
  fxy = fxy / ((x2 - x1) * (y2 - y1))
  
  GetBilinearInterpolation = fxy
  
End Function
Public Sub GetNeigbourIndices(inArr As Variant, x As Double, ByRef lowerX As Single, ByRef upperX As Single)
 N = UBound(inArr, 1)
 If x <= inArr(1, 1) Then
    lowerX = 1
    upperX = 1
  ElseIf x >= inArr(N, 1) Then
    lowerX = N
    upperX = N
  Else
    For I = 2 To N
      If x < inArr(I, 1) Then
        lowerX = I - 1
        upperX = I
        Exit For
      ElseIf x = inArr(I, 1) Then
        lowerX = I
        upperX = I
        Exit For
      End If
    Next I
  End If
End Sub