You have to use conversion units for this. For every 1 mile, there is an equivalent amount of 1,609 meters. For every 1 minute, there is an equivalent amount of 60 seconds. Using the dimensional analysis approach, the solution is as follows:
Acceleration = 15.5 m/s²*(1 mile/1,609 m)*(60 s/1 min)² = 34.68 miles/min²
