"Good thing this city has a metro."
Translation:Onneksi tässä kaupungissa on metro.
not a native, but from what I've gathered... you use -lla for a person that possesses something and -ssa for an object/place that "possesses" something (a possession that doesn't rely on someone)
for example, if you have four tires you would say minulla on neljä rengasta (you have a tire, but you can always throw it away or something) but if the car has four tires you would say autossa on neljä rengasta, you can see that as "there are four ties in a car" which is basically the same thing
and for that reason you should use tässä kaupungissa in that exercise :)