Explanation:
remember the trigonometric triangle in a circle ?
sine is the up/down leg, cosine is the left/right leg.
all we need to consider in a circle with a radius <> 1, that we need to multiply the trigonometric functions by the radius to get the actual side lengths.
the radius is the Hypotenuse (the side opposite of the 90° angle).
y = cos(30)×8 = sqrt(3)/2 × 8 = 4×sqrt(3)
the number in the green box is therefore 4.