Find all files (-type f) that have been modified in the last day (-mtime -1) that do not have the .txt extension (! -name '*.txt') and execute the command 'tar -cvf backup.tar {}' on each of them ( -exec tar -cvf backup.tar {} \;).
find ~ -type f -mtime -1 ! -name '*.txt' -exec tar -cvf backup.tar {} \;