Plural of "abacus"

Solution 1:

As RGW1976 said, both abacuses and abaci are correct. Arabic has nothing to do with the matter: the English word abacus is a direct borrowing of Latin abacus, so it comes with the Latin plural abaci. It also forms the native English plural abacuses. Other examples of this two-plurals phenomenon are the plurals appendixes and appendices, formulas and formulae, and indexes and indices.

In case you’re curious, the Latin word is itself a borrowing of abakos, the genitive of Greek abax 'counting board'. This in turn is probably a borrowing of Hebrew ’ābāq 'dust', so the word may in fact have Arabic relatives, but they have absolutely no bearing on how it forms its plural in English.

Solution 2:

According to Merriam-Webster, "abacus" has two proper plural forms: abaci and abacuses.

Looks like you were both correct!

Personally, I would tend to use your choice of "abacuses" over "abaci" as I think the latter could be confusing to some.