you dont need the halt and you seem to have a bracket mis-match
maybe try...

on *:TEXT:!calc*:#: {
  if (!$2-) { .notice $nick 3,15Sorry, you didnt supply an equasion to calculate }
  else { .notice $nick 3,15Calculating $2- $+ ... 3,15 $2- = $calc($2-)  }
}

but nice work keep it up =)