The final answer is positive as shown in the printed solution. The field is pointing in the +x direction. E field = negative derivative of the potential times unit vector. If potential at x was negative as your are suspecting, and 0 at infinity, the derivative is > 0 and negative derivative is <0 , giving a field pointing in the wrong direction. See the link posted:

Why does this line integral give the wrong sign?

The final answer is written correctly, without a negative sign, as shown in the printed solution. The field is pointing in the +x direction.

$\vec{E} = -\frac{\mathrm{d}U }{\mathrm{d} x} \hat{x}$

If the potential at x was negative as your are suspecting, and 0 at infinity, the derivative is > 0 and negative derivative is <0 , giving a field pointing in the wrong direction. See the link posted:

Why does this line integral give the wrong sign?