What can i use to connect...

... two buildings? The distance between the two buildings is 7KM

In my country, there would be two possibilities:

lease a line between the two points, via a telecommunications provider that offers leased lines, or test if you can establish an effective wireless link, provided there is optical contact between the two buildings.

You could also cooporate with some existing wireless network of your area (such as a free metropolitan network), to help you connect the two places through their nodes (in exchange you'd most likelly have to serve as backbone node as well), but that wouldn't be advisable if you will pass sensitive data.