You should use sin ratio to find the value of x
sin an angle = side adjacent to the angle / hypotenuse
So from the question, we should solve it like this,
cos 60° = x/12
x/12 = cos 60°
x/12 = 1/2
x = 1/2 × 12
x = 6
The value of x is 6, no need to use radical equation because there's no radical