Answer
$a=5$ and $b=6$
Work Step by Step
If $T(\vec{x})=\mathbf{A}\vec{x}=\vec{b}$ where $\vec{x}$ is an $a$-dimensional vector and $\vec{b}$ is a $b$-dimensional vector, then then $\mathbf{A}$ must have $b$ rows and $a$ columns.
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