What is the usage of -9 in kill command ?

There are many switches other than 9 and can be used with kill command.

• SIGHUP (hangup)
  Tells the program that the user disconnected (e.g. SSH session or terminal window was closed)
  Usually does a graceful shutdown of the program.

• SIGINT (interrupt)
  Sent when you hit CTRL+C
  Usually means "stop what you're doing" - may or may not kill the program

• SIGTERM (terminate)
  The default sent by kill and killall.
  Usually terminates the program... sometimes after the program finishes whatever it's doing.

• SIGSTOP (stop)
  Sent when you hit CTRL+Z
  Pauses the program, discussed above

• SIGPIPE (pipe closed)
  Tells the program that the the pipeline has closed.
  Usually terminates the program.
  For example cat /etc/passwd | head -n1 will send SIGPIPE to the cat process after the first line is printed, causing cat to terminate before finishing the whole file.
   

How to add 2 numbers in shell script ?

Use $(expr ..) or $((..))

sum = $(expr "$num1" + "$num2")

echo $sum

OR

sum = $((num1 + num2)) 

echo $sum 

Following are incorrect commands :

echo $num1 + $num2;

echo `$num1` + `$num2`;