An airplane takes off miles south of a city and flies due north at a constant speed of 12 and a half miles per hour. what is the plane’s position relative to the city after 45 minutes?

At a speed of 170mph the plane will go 3/4 of that in 45 minutes (since 45 minutes is 3/4 of an hour)

170 x .75 or 170 x 2/4 = 127.5 miles from where it started.
Since it is 12.5 miles south of the city we can write that as

-12.5 + 127.5 = 115 miles north of the city.
Picture the starting point as being below the city, consider the city to be zero (0) and you will see that the first 12.5 miles of the 127.5 gets you to the city and then the rest gets you 115 miles north of the city.  Hope this helps.