If backfill material is soil and the sheet pile is inserted in a clay, then find out the depth D ?

Question

here in the question z1=3 m 

                                 z2=3 m

we have to find out the depth D in the question . 

 given unit weight =1.9 t/m3

           submerged unit weight=1 t/m3

           submerged unit weight of clay=1 t/m3