we know that
the speed's formula is equal to
[tex]speed=distance/time[/tex]
in this problem we have
[tex]distance=84\ miles\\time=2\ hours[/tex]
Substitute the values in the formula
[tex]speed=84/2[/tex]
[tex]speed=42\frac{miles}{hour}[/tex]
therefore
the answer is
[tex]42\frac{miles}{hour}[/tex]
