A particle moves along the x-axis so that the time ts, its displacement xm from the origin satisfies the differential equation

$\displaystyle \frac{d^2x}{dt^2}+2\frac{dx}{dt}-35x=70t + 31$

Given that when t=0, the particle is at rest at the origin, find its displacement at time ts.

I'm struggling with this because i think it involves complex roots, which i haven't done before.i know the C. Function is $\displaystyle e^{\alpha t}(Acos\beta t + \beta sin \beta t)$

and from the equation i get

$\displaystyle d^2 + 2d - 35$

$\displaystyle \frac{-2^+_- \sqrt{4-140}}{2} $

$\displaystyle \frac{-2^+_- \sqrt{-136}}{2}$

Im a bit stuck and where to go from here. Can i still use surds when its complex? ie $\displaystyle \sqrt{-136}=2\sqrt{-34}$ then i can cancel the 2's with the denominator?