Given : $\mathit{v}=1.5{\mathrm{kms}}^{-1}=1500{\mathrm{ms}}^{-1}$
$\mathit{t}=14\mathrm{}\mathit{s}.$
Let the distance between the source of ultra-sonic wave and the reflector be ' $\mathit{d}$ ' meter.
we know,
$\mathit{v}=\frac{2\mathit{d}}{\mathit{t}}$
$\Rightarrow 2\mathit{d}=\mathit{v}\times \mathit{t}$
$\Rightarrow \mathit{d}=\frac{\mathit{v}\times \mathit{t}}{2}=\frac{1500\times 14}{2}=10500\mathrm{m}$
Total distance covered $=2\mathit{d}$
$=2\times 10500$
$=21000\mathrm{m}$
$=21\mathit{k}\mathit{m}$