A is the transformation matrix. For a transformation T, we need some kind of "transformer" and A is the transformation matrix I used. v is a random vector that is being transformed, in this case it's just the zero vector.
Essentially you are using what you are asked to prove- you can write a linear transformation as a matrix because, among other things, T(0)= 0.
A linear transformation, T, from vector space U to vector space V, must satisfy
1) T(u+ v)= T(u)+ T(v) with u and v vectors in U.
2) T(au)= aT(u) with a a member of the underlying field and u a vector in U.