I am having struggles making this calculator script. help?

function onChatted(msg, recipient, speaker)

local source = string.lower(speaker.Name)
      msg = string.lower(msg)

if (string.match(msg, "what is 5x5")) then
print(answer)
end

how would i go for making something like this i do not want it to do only 5x5 but other calculations aswell :/

p.s I dont have good knowledge on string manipulation, if you do it'd be very appreciated if you could help me out.