If you are in college, this question is likely done with calculus.
a = 12
k = 0.2
v(x) = 0 at the maximum after differentiation
V(x) = kx(a - x)
The easiest way to do this is to remove the brackets.
V(x) = akx - kx^2 Substitute the constants
V(x) = 12*0.2*x - 0.2x^2
V(x)/dx = 2.4 - 0.4x = 0 at the maximum
2.4 - 0.4x = 0
0.4x = 2.4
x = 2.4/0.4
x = 6
If you don't know any calculus yet, you can do this by completing the square.