hi sir/ma'am how to convert a char to a string?

i got this error.

sketch_aug27a:90: error: invalid conversion from 'char' to 'const char*'
sketch_aug27a:90: error: initializing argument 1 of 'String::String(const char*)'
sketch_aug27a:93: error: invalid conversion from 'char' to 'const char*'
sketch_aug27a:93: error: initializing argument 1 of 'unsigned char String::equals(const char*) const'

char x;
n[20];

     
      if((x, n)>0)   // Number, Number length
      {
      String num=x;
      if(num.equals("12345")){
      Serial.print("Ok");
     }
     else{
     Serial.print("Done");
}

 

I think you need

String num = String(x, DEC);

see http://arduino.cc/en/Reference/StringConstructor