How do you represent a SOP in terms of NAND?
Do following procedure:

Step 1: Convert all NOT-operators

Step 2: Convert all AND-operators (left to right)

Step 3: Convert all OR-operators (left to right)

How to do it?:

Convert NOT-operator: X¯=X⊕X

Convert AND-operator: X⋅Y=(X⊕Y)⊕(X⊕Y)

Convert OR-operator: X+Y=(X⊕X)⊕(Y⊕Y)