int a = 0, b = 0, x = 0, y = 0, c = 0;
private void Button1_Click(object sender, EventArgs e)
{
a = Convert.ToInt32(textBox1.Text);
b = Convert.ToInt32(textBox2.Text);
x = Convert.ToInt32(textBox3.Text);
if(x%2==0)
{
c = 2 * a + b;

}
else
{
c = a + 3 * b;
}
MessageBox.Show(c.ToString());

}