The gun is fired vertically upward. Bullet muzzle velocity v. At what height will the bullet speed decrease by 2 times?

On the y-axis: y = y0 + v0yt + ayt ^ 2/2
y = h, y0 = 0, v0y = v, ay = -g
Then:
h = vt-gt ^ 2/2
2— Equation of speed:
vk = v0 + ayt, Where vk = v / 2, v0 = v, ay = -g
Then:
v / 2 = v-gt
2gt = v
t = v / 2g Substitute in the 1st, we get
h = v ^ 2 / 2g-v ^ 2 / 8g = 3v ^ 2 / 8g
And here’s the answer: h = 3v ^ 2 / 8g