Answer
$264Hz$ or $258Hz$
Work Step by Step
The formula for the beat frequency is $$f_{beat}=|f_2-f_1|$$ If the beat frequency is 3 beats per second, that means the distance between the piano frequency and $261Hz$ must be three, either 3 hertz below or 3 hertz above. In this case, the frequency would be either $261+3=264Hz$ or $261-3=258Hz$.
