How do I call a function from an rdlc report in VB.NET

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VB

Hello
I am currently developing a project in VB.Net and MYSQl Server and I got stuck:
My problem is that i have created an rdlc report with two text boxes. Text box 1 display amount in numbers (eg 3000)by using

=sum(Fields!Total_Amount.Value)

and i want textbox 2 to display the same number in words(eg
three thousand only) by using the a function numtowords saved in module.
How can i do that.

The function numtowords is working fine on a form in vb.net.
Regards
Sarfaraz

What I have tried:

I tried something like =ReportItems!txt1.value=numtowords(ReportItems!txt2.value)
but it did not work.