Newbie Question: How Do I Show Rows From The 14 Last Days? (using Sqldatasource)

I am using sqldatasource to make a connection to my access database. I have a table with a field called "last_activity" (Date/Time datatype).
I want to make a query which shows users/players (the rows in the table) that have been active the last 14 days, so I have to use "last activity" as parameter.
But how do I code this?

<asp:SqlDataSource runat="server" id="race_a_car_Source" ProviderName="<%$ ConnectionStrings:speedracerConnectionString1.ProviderName %>" ConnectionString="<%$ ConnectionStrings:speedracerConnectionString1 %>" SelectCommand="SELECT * FROM [race_a_car] WHERE ([last_activity] &gt;= ?)">
<SelectParameters>
<asp:parameter DefaultValue=" what should I write here? " Name="last_activity" Type="DateTime" />
</SelectParameters>
</asp:SqlDataSource>