Check the picture below.
so, using the 30-60-90 rule we can see the length on either the left or right side is 7.5 ft, now, the shaded section is how long "y" is, we can see it is not 7.5, since it doesn't go all the way up in the bottom triangle, since half of 7.5 is 3.75 and the shaded part is well above the half, then is not 3.1 either, now we can also say that 4.3 is close to 3.75 or close to the half, but the shade is higher than that, so we can say it may not be 4.3, so the only choice leftover is 5.5, or the "y" distance is just 2 ft shy from the top of the triangle, and that's feasible.