$o You notice a one$-$dimensional$-$thinking snail crawling along one rail of a railroad track. Naturally, you move it to a safer place.

But before you do$,$ you observe it as it moves from position $x_i=-85\mathrm{.}9cm$ to position $x_f=-54\mathrm{.}5cm$ along the rail, as measured from a nearby joint between two segments of rail.

What is the magnitude $x$ of the snail's displacement in centimeters?