on the other hand, the word "which" does not introduce a restrictive clause. it introduces a nonrestrictive (or parenthetical) clause. A nonrestrictive clause only provides additional information that is not essential to the meaning of the sentence. in sentence (3) the phrase "which is positive" simply provides mor information about the solution. this clause may have been added because the solution to an earlier equation is negative. in fact, it would be more appropriate to add a comma.