Concept:
The composition \(T_2T_1\) means
\[
T_2T_1=T_2\circ T_1
\]
So we first apply \(T_1\), then apply \(T_2\).
Step 1: Apply \(T_1\).
\[
T_1(x,y,z)=(3x,4y-z)
\]
This gives an ordered pair in \(R^2\).
Step 2: Apply \(T_2\).
The transformation \(T_2\) is
\[
T_2(u,v)=(-u,v)
\]
Here,
\[
u=3x,\qquad v=4y-z
\]
Therefore,
\[
T_2(3x,4y-z)=(-3x,4y-z)
\]
Step 3: Final answer.
\[
\boxed{(-3x,4y-z)}
\]