latex comand \cancelto

Why the comand \cancelto is not working in online equation editor?

We need to install an additional package called cancel. I image you're after something like this: http://anrg.usc.edu/~joonahn/www/index.php/Canceling_Terms_in_LaTeX_Math

We have to thoroughly test new packages before we can take them live; so this will take a few weeks.