It's not ambiguous, the operation of dividing one matrix by another is not defined.
I disagree with BobP. "Dividing", in any group, field, or ring, means multiplying by the inverse and is defined in a vector field.
The "ambiguity" is in A/B, and the reason "multiplying by the inverse" is never written that way, is that multiplication of matrices is not commutative. That could mean either or .