Email form validate a simple sum ? ? ?

[Websiteking]

I have '2+7' on my email form and want it to validate the answer given by the user is 9, I have a validation javascript that checks the user's email is good and that they have filled in their name and other details but what I need is for it to check that the answer to my sum is correct (9)..I'm doing this to discourage Spam.

I cant use 'Captcha' as Im with 123-reg and they dont host PHP