Answer:
12
Explanation:
Differentiate the equation. It will give you -1/6x + 2 = dy/dx.
Let dy/dx = 0 (you do this because the highest point is usually the turning point. The gradient is zero at the turning point, meaning that you'd have to set dy/dx to be zero to figure out the highest point).
Then rearrage for x
-1/6x+2=0
1/6x = 2
x = 12