How to change one character to string in string variable

I have string "2x+3" and I want change 'x' to string "8".

String operation = "2x+3";
String x = "8";

And I want result such as "2*8+3"

or

String operation = "x+3";
String x = "8";

And I want result such as "8+3"

Does anyone know how to solve ?