Calculating Values Of Several Equations In Application

I have to calculate values of several equations in an application. For example, one these equations could be:

d = a + b*z + c*z (this is what is being displayed to the user, assume that 'a', 'b' and 'c' are provided as textboxes, assume that z is a hardcoded value for now)

Now, the user could enter a=1.5, b= -0.5 and c = -0.6 (or even 'a' could be negative). So, I just tried a sample code for simple values but to no avail...

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
Dim val1, val2, val3_calculated, first, second As String
first = "8"
second = "3"
Dim arr(10) As String
[Code] .....

Error:
But it throws an error saying that I am not using any databind control.

Concerns:
First of all, is it necessary to use this "eval" operator in here? Is there any way to keep "-0.5" with the sign in asp.net using vb?