Displaying Input into textbox - vb.net

Question

I am trying to display a calculation into a TextBox, but having trouble with getting it to show. I want it to show once all input fields are true.
Public Class VehicleAudit
Private Sub Calculate()
Dim validMiles As Boolean = False
Dim validPercent As Boolean = False
Dim validAvg As Boolean = False
Dim inputtedMiles As Double
Dim inputtedPercent As Double
Dim inputtedAvgCost As Double
Dim servTruck As Integer
Try
inputtedMiles = Double.Parse(txtMilesDriven.Text)
inputtedPercent = Double.Parse(txtPercent.Text)
inputtedAvgCost = Double.Parse(txtAvgCost.Text)
Catch ex As FormatException
MessageBox.Show("Please enter all values and try again")
Return
End Try
Dim cal As String = FormatCurrency(Convert.ToString(inputtedAvgCost * inputtedMiles * (1.0 + inputtedPercent))) + " dollars."
ValidateBoxes(inputtedMiles, 0, 10000, "Miles must range from 0-10000.", validMiles)
ValidateBoxes(inputtedPercent, 0.1, 0.5, "Please enter percent from .10 to .50", validPercent)
ValidateBoxes(inputtedAvgCost, 0.25, 0.75, " Please enter Average cost from .25 to .75", validAvg)
If (validAvg And validMiles And validPercent) Then
Dim totalCost As Double
If boxVehicleSelect.SelectedIndex = 9 Then
servTruck = inputtedMiles / 100 'this way we lose precision using the integer, so values below 100s are dropped.
totalCost = servTruck * 15.46
Else
totalCost = inputtedAvgCost * inputtedMiles * (1.0 + inputtedPercent)
End If
End If
End Sub
Private Sub txtTotalCost_TextChanged(ByVal Calculate As String, e As EventArgs) Handles txtTotalCost.TextChanged
End Sub

Answer 1

You appear to already have a block that runs when all three values are "valid". Simply output that value at the bottom of it:
If (validAvg And validMiles And validPercent) Then
Dim totalCost As Double
If boxVehicleSelect.SelectedIndex = 9 Then
servTruck = inputtedMiles / 100 'this way we lose precision using the integer, so values below 100s are dropped.
totalCost = servTruck * 15.46
Else
totalCost = inputtedAvgCost * inputtedMiles * (1.0 + inputtedPercent)
End If
' Output the computed "totalCost" some where.
' Here I'm using a Textbox called "txtTotalCost":
txtTotalCost.Text = totalCost.ToString()
End If
Edit...
Also call your Calculate() method whenever one of your textboxes changes:
Private Sub TextChanged(sender As Object, e As EventArgs) Handles txtMilesDriven.TextChanged, txtAvgCost.TextChanged, txtPercent.TextChanged
Calculate()
End Sub
Note how all three textboxes are listed after the handles keyword.