Answer
See below
Work Step by Step
Let $u$ be arbitrary vectors in $U$, and let $c$ be a scalar.
We obtain:
$(T_2T_1)(cu_1)=T_2(T_1(cu_1))\\
=T_2(cT_1(u_1))\\
=cT_2(T_1(u_1))\\
=c(T_2T_1)(u_1)$
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