Converting Boolean Expression for NAND Gate Implementation - DeMorgan's Law
You have the basic rule that (a or b) = not (not a and not b), so you use that repeatedly. The first application gave you B~CD + ACD = NAND(~(B~CD), ~(ACD)). You're almost there: 

NAND(~(B~CD), ~(ACD)) = NAND (NAND (B, ~C, D), NAND (A, C, D))
 = NAND (NAND (B, NAND (C, C, C), D), NAND (A, C, D))