Answer
$\tau = 8.5\times 10^{-22}~N\cdot m$
Work Step by Step
$p$ is the dipole moment
$E$ is the electric field
Note that when the dipole moment is perpendicular to the electric field, the angle $\theta$ between the directions is $\theta = 90^{\circ}$
We can find the magnitude of the torque on the dipole when the dipole moment is perpendicular to the electric field:
$\tau = p \times E$
$\tau = qd~E~sin~\theta$
$\tau = (2)(1.6\times 10^{-19}~C)(0.78\times 10^{-9}~m)(3.4\times 10^6~N/C)~sin~90^{\circ}$
$\tau = 8.5\times 10^{-22}~N\cdot m$
