Change units:
1 mile = 1609.34 m = 1609.34 * 10^6 micro m = 1.60934 * 10^9 m
1 hour = 3600 s = 3600 * 10^9 nano seconds = 3.6*10^12 nano sec
3 miles / hour = 3 * 1.60934 * 10^9 / (3.6*10^12) = 0.0013411 micro m/nano second = 1.3411*10^(-3) um / ns
Check it! Best
