Visual Basic - xpath para obtener datos xml

Hola, alguien puede explicarme como hacer consultas xpath ?
necesito obtener el <trackingnumber> dentro de <trackingnumbers>

<?xml version="1.0" encoding="windows-1252"?>
<OpenShipments xmlns="x-schema:OpenShipments.xdr">
<OpenShipment xmlns="x-schema: OpenShipments.xdr" ShipmentOption="" ProcessStatus="Processed"><ShipTo><CustomerID> 27683 </CustomerID>
<CompanyOrName> NAZRUL ISLAM </CompanyOrName>
<Attention> (LOCUTORIO - INTERNET ) </Attention>
<Address1> C MESON DE PAREDES, 72 BJS. LOCAL 3 </Address1>
<Address2> 10:00-23:00 </Address2>
<Address3> 06489-1 </Address3><CountryTerritory>ES</CountryTerritory>
<PostalCode> 28012 </PostalCode>
<CityOrTown> MADRID </CityOrTown>
<Telephone> 634048290 </Telephone></ShipTo>
<ShipmentInformation>
<ServiceType> ST </ServiceType>
<NumberOfPackages> 1 </NumberOfPackages>
<DescriptionOfGoods> ENTREGAR ROLLOS </DescriptionOfGoods>
</ShipmentInformation>
<Package>
<Weight> 2 </Weight></Package>
<ProcessMessage>

<ShipmentRates>
<ShipmentCharges>
<Rate>
<Published>12,18</Published>
</Rate>
</ShipmentCharges>
<ShipperCharges>
<Rate>
<Published>12,18</Published>
</Rate>
</ShipperCharges>
<ReceiverCharges>
<Rate>
<Published>0,00</Published>
</Rate>
</ReceiverCharges>
<PackageRates>
<PackageRate>
<TrackingNumber>1Z5F29F96856401422</TrackingNumber>
<PackageCharges>
<Rate>
<Published>0,00</Published>
</Rate>
</PackageCharges>
<Delivery_AreaSurcharge>
<Rate>
<Published>0,00</Published>
</Rate>
</Delivery_AreaSurcharge>
</PackageRate>
</PackageRates>
</ShipmentRates>
<TrackingNumbers>
<TrackingNumber>1Z5F29F96856401422</TrackingNumber>
</TrackingNumbers>
<ImportID></ImportID>
<Reference1></Reference1>
<Reference2></Reference2>
</ProcessMessage>
</OpenShipment>

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