A researcher compares the mean weekly hours spent on exercise for residents of two towns. A random sample from Town A has size $n_1=18$, mean ${\bar{x}}_1=12.4$ hours, and standard deviation $s_1=3.1$. A random sample from Town B has size $n_2=22$, mean ${\bar{x}}_2=9.7$ hours, and standard deviation $s_2=2.8$. Assume both populations are approximately normal with equal variances. Construct a $90\%$ confidence interval for ${\mu }_1-{\mu }_2$.