Bonjour! In French, how do you know in math equations when to use "font" or "égale" for equals?

I had to complete a worksheet where it marked me wrong for writing "Neuf cent soixante-six moins trois cent quarante-deux font six cent vingt-quatre." It told me the right answer was the exact same sentence, but with "égale" instead of "font."

However, it marks "Deux cents plus trois cents font cinq cents" correctly.

Can someone please explain why and when to use each word?

Correct Answer is