You walk 2 miles in 12 hour means your rate is:

v = 2/12

v = 1/6 mph

d = 3 mi the distance to walk

t = the time of the 3 mile walk

since v = d/t we have:

3/t = 1/6

........

click here to see the equation solved for t

........

t = 18 hour

It will take 18 hours to make the 3 miles walk.