Answer
See below
Work Step by Step
For every $v \in V$ there exist scalars $a_1,a_2,...,a_k$ such as:
$v=a_1v_1+a_2v_2+...+a_kv_k$
We obtain:
$T(v)=T(a_1v_1+a_2v_2+...+a_kv_k)\\
=a_1T(v_1)+a_2T(v_2)+...+a_kT(v_k)\\
=a_1S(v_1)+a_2S(v_2)+...+a_kS(v_k)\\
=S(a_1v_1+a_2v_2+...+a_kv_k)\\
=S(v)$
with $v \in V$
Hence, $T=S$