You're trying to find a distance per given time, so by creating a ratio of the first time and setting it equal to an incomplete ratio of the second time, you can solve it easily :)
[tex] \frac{570~miles}{15~hours}=\frac{x~miles}{7~hours} [/tex]
Cross-multiply the fractions:
[tex]570*7=15*x\\3990=15x\\x=\frac{3990}{15}=266~miles[/tex]
