Yes I got the solution by myself.I did something like this in my xslt sheet:
<xsl:variable name="fid" select="./ns1:AirItinerary/ns1:OriginDestinationOptions/ns1:OriginDestinationOption/ns1:FormData/ns1:FBC/@FlightNumber"> </xsl:variable>
<a href="AirPrice.aspx?fid={$fid}">BookNow</a>