Answer: 1848 feet per minute
Step-by-step explanation:
[tex]\dfrac{21\ miles}{1\ hour}\times \dfrac{1\ hour}{60\ minutes}\times \dfrac{5280\ feet}{1\ mile}\quad =\dfrac{21\times 5280\ feet}{60\ minutes}\quad =\dfrac{110,880\ feet}{60\ minutes}\\\\\\=\large\boxed{1848\ feet\ per\ minute}[/tex]