Is ∑ with negative value solvable? Is it possible to have a negative value in sigma?
e.g.
$y = \Sigma_{k=0}^{k=-2} k \times 10$   
Will this give the result $(0 \times 10) + (-1 \times 10) + (-2 \times 10) = -30 $?
Or will it be $\infty$ because $k$ will be increased with $1$ until it equals $-2$ (which is never).
Or something else?

Yes we can have a negative value in sigma.
You just need to write it like this:
$$y = \Sigma_{k=-2}^{k=0} k\times 10$$
