how to call connection string from web config file.?

<configuration>
<appSettings/>
<connectionStrings>
  <add name="sunrockConnectionString" connectionString="Data Source=WINXP_PC\SQLEXPRESS;Initial Catalog=sunrock;Integrated Security=True"
   providerName="System.Data.SqlClient" />
 
 </connectionStrings>
<system.web>

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cn.ConnectionString = ConfigurationManager.ConnectionStrings["sunrockConnectionString"].ToString();
        cn.Open();