What have you tried? The definition of "linear transformation" is, of course, that f(au+ bv)= af(u)+ bf(v) for any scalars a and b and vectors u and v. Here the "vectors" are two by two matrices.

Is P(aM+ bN)P^(-1)= aPMP^(-1)+ bPNP^{-1}?

Is T(af(t)+ bg(t))= aT(f(t))+ bT(g(t))?

A linear transformation is an isomorphism if and only if it is one-to-one.

If T(M)= T(N), does it follow that M= N?

If T(f(t))= T(g(t)), does it follow that f= g?