Can I use string and int in Serial.println together?
The answer by canadiancyborg is fine. However, it is always better to avoid using String objects if at all possible, because they use dynamic memory allocation, which carries some risk of memory fragmentation. Simple and safe:

int answer = 42;
Serial.print("The answer is ");
Serial.println(answer);


The only drawback is that the constant string ("The answer is ") takes some RAM. This can be avoided by embedding it in the F() macro, which means “keep this in Flash memory only”. Then, the most memory-friendly idiom is:

int answer = 42;
Serial.print(F("The answer is "));
Serial.println(answer);