Answer
slope$=-1$
See graph.
Work Step by Step
1. Find the derivative of $f(x)=3x^2-x$ as $f'(x)=6x-1$.
2. The slope of the tangent line is the same as the derivative at $x=0$, we have $m=6(0)-1=-1$
3. The tangent line is $y-0=-(x-0)$ or $y=-x$
4. See graph.