Speed formula:
[tex]s=\frac{d}{t}[/tex]
d is the distance
t is the time
As you need to find a time having the distance and speed, solve the equation above for t:
[tex]\begin{gathered} t\cdot s=d \\ t=\frac{d}{s} \end{gathered}[/tex]
Use the given data to find the time:
[tex]\begin{gathered} t=\frac{278mi}{38mi/h} \\ \\ t=7.31h \\ \\ t=7h \end{gathered}[/tex]
Then, it would take you 7 hours to drive 278 mi at 38mi/h
