Force on a charge obtained from electric potential

[Alan I]

Homework Statement

In a certain region of space, the electric potential along the x-axis is given by:

V = 111x3 - 517x - 258 where V is in Volts and x is in meters.

Find the force F (magnitude in N and direction) felt by a -444 μC point charge at point (56.5 cm,0). (For direction, all you need is the sign.)

Homework Equations

E=dV/dx
F=qE

The Attempt at a Solution

[/B]
dV/dx=d/dx(111x³-517x-258=333x²-517-258=E

⇒ E_(0.565m)=-668.7 N/C

⇒F=qE=(-444*10^-6)*(-668.7)≅0.297

The answer turned out to be wrong so if someone can give me a hint towards where my mistakes are I would greatly appreciate it!

 

[Orodruin]

What is the derivative of 258 with respect to x?

 

[Alan I]

What is the derivative of 258 with respect to x?

Thanks!