Question

how to find the unit rate of 346 mi on 22 gal

Answer 1

The unit rate is when 1 is in the denominator, we have to divide the numerator and denominator by the denominator in order to get 1.
346   ÷22        15.72
-----           =  --------- 
22    ÷22            1 
Therefore the unit rate would be 15.72:1

Answer 2

Divide 346 mi by 22 gal.
The quotient will be "miles per gallon", which is exactly what you want.