Answer
$m\widehat{TBD}=180^{\circ}$
Work Step by Step
We must know that the measure of a semi-circle arc is equal to $180^{\circ}$
In the given question, we can see that $\widehat{TBD}$ is a semicircle.
This implies that
$m\widehat{TBD}=180^{\circ}$