[Boldewyn]

As a non-native speaker, is “sum 7x8” an error? Shouldn’t it be “product 7x8”?

[TomNomNom]

In (British) English, equations are sometimes called 'sums'. It's a confusing colloquialism, and you're right to be confused by it.

[sdab]

American vs. British English usage. sum is an acceptable general term in British usage. In American English, it would indeed need to be product.