Tinkster is 100% correct. But just to clarify a bit:
1. Escape sequence:
printf(( " Calculated enthalpy is %d btu per\lb\n",enth));
enthalpy.c:74:15: warning: unknown escape sequence '\l'
C/C++ (and Java and C#) have a notion of "escape characters": special
codes like '\t' (tab), '\b' (backspace), '\n' (newline in Unix, CR/LF in DOS), and so forth.
There are a fixed, predefined set of these escapes, and the compiler message
is a complaint telling you "\lb" isn't one of them.
So the question is "What did you *intend* to do?"
If you wanted to print the text "per\lb", then Tinkster's advice is correct:
printf(( " Calculated enthalpy is %d btu per\\lb\n",enth));
<= THE "\\" SYNTAX ESCAPES THE ESCAPE CHARACTER, GIVING YOU A SINGLE "\"
2. End of file:
Many (although not necessarily all) C/C++ compilers demand a "\n" (linefeed, or CR/LF) after
the last line of code. So it's always good practice to put an explicit line or two after the last line in your source file. Tinkster said "before" the brace; I think he meant "after"...