private String ans;
    private Double answer;
    private Double answerPlus;

    private void btnM_Click(object sender, EventArgs e)
    {
        ans = txtDisplay.Text;
        answer = double.Parse(ans);
        lblblank.Text = answer.ToString();
    }

    private void lblblank_MouseEnter(object sender, EventArgs e)
    {
        lblblank.Show();
        lblblank.Text = answer.ToString();
    }

    private void lblblank_MouseLeave(object sender, EventArgs e)
    {
        lblblank.Hide();
    }

    private void btnMplus_Click(object sender, EventArgs e)
    {
        answerPlus = answer + double.Parse(ans);
        lblblank.Text = answerPlus.ToString();
    }