You need to use conversion factors. First change meters to miles and then seconds to hours
[tex]15 \frac{meter}{sec} [/tex]
[tex]15 \frac{meter}{sec} * \frac{1 \space mile}{1609 meters} =[/tex]
[tex]\frac{15\space miles}{1609} = \frac{9.32 miles}{sec}[/tex]
[tex]\frac{9.32 miles}{sec} * \frac{60sec}{1hour} = \frac{559.2 miles}{hour}[/tex]