Question on Negation Normal Form

Determine the NNF of the following formulas:

1. r1 = (NOT(X1 + X3) * (NOT(X2) * X3)) * NOT(X2)

2. r2 = NOT(X1 + NOT(X2)) + NOT(X3)

3. r3 = NOT(NOT(X1))

4. r4 = NOT(X1) AND (NOT (X2 OR X3))