Work done by a particular force on a particle in one dimension

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I've been working on this problem a while.

I am supposed to prove that, for a particle of mass $m$, which is subject to a force $\vec{F}$ = $-av^2\vec{\hat{v}}$, (with $a$ a positive constant) the total work done in one-dimension as the particle moves a distance $b$ is:

$W = \frac{1}{2}mv_0^2(e^{-2ab/m} - 1)$

where $v_0$ is the initial velocity.

I have obtained an equation for $v$ in terms of the direction $x$, which I think is right.

$v = e^{-ax/m}$

so $F = -ae^{-2ax/m}\vec{\hat{v}}$ and $d\vec{s} = dx$. Using all this, I get a similar answer but without the $v_0^2$. Can someone tell me where I went wrong?

Thank you