linear extrapolation to abscissa

[e-letter]

>1. get [a, b] in y = ax + b
Entering this into the scilab command resulted in error 276
>
>   coefs = regress(x, y);
>
>2. calculate x for y = 0 from
>   y = ax + b
>   x = (y-b)/a
>
>with y = 0:
>   x = -b/a
>that is
>   x = -coefs(1)/coefs(2)

Please explain in full detail; I do not understand any of what you wrote!