Unit Circle: Quadrant

Question

The point P is on the unit circle. If the y-coordinate of P is 1/2, and P is in quadrant II, then:

x=

Can I have an explanation? It's a bit confusing to me.

Answer 1

Answer 2

Quadrant II means x is negative and y is positive.

On the unit circle means x² + y² = 1,

which means that x² = 1 – y²

If y = ½ the x² = 1 – (½)² = 1 – ¼ = ¾

x² = ¾ and x is negative means that

x = –√3/2