So normally you would take the amount of miles and divide it by the hours to get a simple miles per hour.
Division of two fractions can be made easier by flipping the second fraction, and changing the operation to multiplication.
(1/6) / (1/42)
= (1/6) x (42/1)
From then, it is simply top x top, bottom x bottom.
= 42/6
Complete the division
= 7/1
= 7 miles per hour