Delta(f) =f0 * v_rel/c , where v_rel is the relative velocity between source and observer.
The source is moving at a speed v but the sound bounced off the wall and the receiver is moving toward it too.
So,
Delta(f) = f0 (2 V) / c
Hence
V = c Delta(f) /(2 f0)= 344 m/s * (7.00Hz/(2*245Hz)) =4.91 m/s