To calculate the Nth root of K you can simply use the formula:

    root = K1/N

The following code gets the numbers, calculates the root, and checks the result.

// Calculate the root.
private void btnCalculate_Click(object sender, EventArgs e)
{
    double number = double.Parse(txtNumber.Text);
    double root = double.Parse(txtRoot.Text);

    // Calculate the root.
    double result = Math.Pow(number, 1 / root);
    txtResult.Text = result.ToString();

    // Check the result.
    double check = Math.Pow(result, root);
    txtCheck.Text = check.ToString();
}