i just need my worked checked on this one.

y=x^4 + bx^2 + 8x + 1 has a horizontal tangent and an inflection point for the same value of x. what must be the value of b?
a: -6
b: -1
c: 1
d: 4
e: 6

the answer i got was a:-6

Question

i just need my worked checked on this one.

y=x^4 + bx^2 + 8x + 1 has a horizontal tangent and an inflection point for the same value of x. what must be the value of b?
a: -6
b: -1
c: 1
d: 4
e: 6

the answer i got was a:-6

ganeshie8 · Answer

http://www.wolframalpha.com/input/?i=4x%5E3%2B2bx%2B8%3D0%2C+12x%5E2%2B2b%3D0

ganeshie8 · Answer

b = -6 is $\large \color{red}{\checkmark}$

anonymous · Answer

thank you