w shape bent frame which is preferably made of metal material having strength and elasticity such as aluminum in the embodiment the first movable socket 31 a has a horseshoe shape portion 311 a and two guiding portions 312 a while the second movable socket 32 a has a horseshoe shape portion 321 a two guiding portions 322 a and two guiding extension portions 323 a wherein the two guiding extension portions 323 a are further formed between the horseshoe shape portion 321 a and the two guiding portions 322 a for elongating a distance between the horseshoe shape portion 321 a and the two guiding portions 322 a therefore when installing the fastening assembly 3 the second movable socket 32 a is firstly in contact with the second fixed holder 12 then the first movable socket 31 can be aligned with the first fixed holder 11 as shown in fig2 by the guidance of the two guiding extension portions 323 a so as to simultaneously finish the engagement operations in another embodiment the guiding extension portions not shown may be formed on the first movable socket 31 a in comparison with the first embodiment of the present invention which needs to simultaneously align the two movable sockets with the two fixed holders the fifth embodiment of the present invention can generate a guidance function so that the alignment operations of the two movable sockets and the two fixed holders can be done in turn and easily carried out referring now to fig8 a a perspective view of.