Yes. The term ST is, in fact, a multiplication as Plato pointed out. You really do need to write it as \(\displaystyle S \circ T\).$$ST\begin{bmatrix}x\\y\end{bmatrix}

=S\left(T\begin{bmatrix}x-4y\\3x\end{bmatrix}\right)

=\left[\begin{array}{c}2(x-4y)+3x \\ x-4y-3x \end{array}\right]$$

do you mean this?

On the other hand I've never seen the composition symbol used in any of the Physics classes I've taken. So if you are doing Math do it as Plato (or your instructor) says. Otherwise ask your instructor.

-Dan