jika f(x) = x2-(k+1)x-40, memiliki kesamaan sumbu simetri x=3, tentukan nilai k

y = ax^2 + bx + c.
persamaan sumbu simetri x = -b/2a
- -(k+1)/2.1 = 3
k+ 1 = 6
k = 6 - 1
k = 5