Answer: 2 cm
Explanation:
Because the two right triangles formed by the dashed segment are congruent (which can be proven by ASA), we know that we have a 30 60 90 triangle where the hypotenuse has a length of 4 cm and we need to find the length of the segment opposite the 30-degree angle.
Since the length of the opposite the 30 degree angle is half the length of the hypotenuse in a 30 60 90 triangle, we know that y = 4/2 = 2.