Code:
TextBox1.Text = "line 1" & Environment.NewLine
TextBox1.Text &= "line 2" & Environment.NewLine
TextBox1.Text &= "line 3"

Dim number As Integer = 42
Dim result As String = "The ultimate answer is " & Cstr(number)

TextBox1.Text &= Environment.NewLine & Environment.NewLine & result
Your TextBox should display this:

line 1
line 2
line 3

The ultimate answer is 42