WA$=5($a$+$c$)$kN$,$ WB$=3($a$+$c$)$kN$.$ The weight of the object D$,$ which will cause A and B blocks to slide together, is D$1,$ and we are asked to find it$.$ The weight of the object D$,$ which will move A and B blocks separately, is D$2,$ and we are asked to find it$.$It would be better if you express the result in terms of A and C$.$However$,$ if necessary, here is a$=4$ and c$=9.$