Passing Value from Form2 to Form1

I have this button in Form2 that when is clicked, a string value is passed back to Form1 and it is displayed in a textbox.
public partial class Form2 : Form
{
double a, b, ave;
string stype;
Form1 frm = new Form1();

public Form2(double x, double y)
{
InitializeComponent();
a = x; b = y;
ave = (a + b) / 2;
}

...

private void button2_Click(object sender, EventArgs e)
{
frm.textBox6.Text = stype;
}
textBox6 Modifiers is already Public.