First line will throw compilation error, as you are trying to store a double value in string.
string amount = Convert.ToDouble(56);
this line should be :
double amount = Convert.ToDouble(56);
Result : 56.0
and second line :
string chkAmount = String.Format("{0:f}", amount)
It will return : 56.00
For detail knowledge of
String.Format
please refer :
String Format for Double [C#][
^]
Good luck.