I'm having trouble with this proof we need to learn -

Let E = µ(I). Prove that µ(A) = EA for every A ∈ M_{nn}

A hint some guy gave is to begin with the ij entry of EA and prove that it equals the ij entry of µ(A), by using the

deﬁnition of matrix multiplication and the deﬁnitions of E and µ(A).

Any help? Thanks a bunch