# Undefined control sequence \coloneqq

I'm trying to export a PDF of a LyX document (it has never caused a problem before). Below I have posted pictures of the error message along with the equation LyX highlighted for me. I don't know what \hbox is, and I didn't use that in my code. Can someone please help me edit the equation so that I can export a PDF?

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This \hbox thing is just an example for a typo, has nothing to do with your text! –  vaettchen Feb 1 at 16:57

The command \coloneqq is unknown, you need to load a package for this, such as mathtools, txfonts or pxfonts.

\usepackage{mathtools}

LyX 2.1 will have direct mathtools support.