Question:
Matrix multiplication is over matrix addition.
Solution:
Matrix multiplication is distributive over matrix addition.
Let $A, B$ and $C$ be three matrices. Then
(i) $A(B+C)=A B+A C$, whenever both sides of equality are defined
(ii) $(A+B) C=A C+B C$, whenever both sides of equality are defined
Matrix multiplication is distributive over matrix addition.