Other router to bridge Airport Extreme ...

I use a Airport Extreme as my main router. I want to improve it's wireless signal because in my father's room it is a little weak because of the arquitecture of the apartment. I have other Linksys router that I am not using (it is a 802.11n too). Can I use it as a bridge to improve the signal of my Airport Extreme? Do I have to connect it with the Ethernet cable to the Airport?

P.S.: What is gateway?

Thanks.

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