<image>As shown in the figure, in rectangle \(ABCD\), point \(E\) is on side \(AB\). △\(EBC\) is folded along the line \(CE\), so that point \(B\) falls on point \(B′\) on side \(AD\). When the folded figure is unfolded, if the perimeter of △\(AB′E\) is 4 cm and the perimeter of △\(B′DC\) is 11 cm, then the length of \(B′D\) is _________ cm. 