The negative sign is shorthand symbol for "opposite". So the answer is the opposite of -1/3 = 1/3. This is because it is a value of the same distance from zero on a line scale where zero represents the origin. Imagine a horizontal line (as this scale) where the middle of the line is a point equal to zero. This line extends infinitely to the left and infinitely to the right. -1/3 lies to the left of zero a magnitude of 1/3. And positive 1/3 lies to the right of zero the same magnitude but on the opposite side(to the right). Hope that helps.
