S is linear so S(kx) = kS(x) and S(x+y) = S(x) + S(y)
T is linear so T(kx) = kT(x) and T(x+y) = T(x) + T(y)
Now
T(S(kx)) =
= T(kS(x)) (since S is linear)
= kT(S(x)) (since T is linear)
and so scalars, k, can be pulled out of T(S(x))
Similarly, T(S(x+y)) =
= T(S(x) + S(y)) (since S is linear)
= T(S(x)) + T(S(y)) (since T is linear)
and so T(S(x)) is a linear transformation.