Answer
Point $A$ is on the bisector of $\angle TXR$ because the two angles that make up $\angle TXR$, which are $\angle AXT$ and $\angle AXR$, are congruent.
Work Step by Step
Point $A$ is on the bisector of $\angle TXR$ because the two angles that make up $\angle TXR$, which are $\angle AXT$ and $\angle AXR$, are congruent.