the point-slope form is ...

y - y<sub>1</sub> = m(x - x<sub>1</sub>)

the given point is (x<sub>1</sub> , y<sub>1</sub>) = (-1, 0)

substitute ...

y - 0 = m[x - (-1)] = m(x + 1)

see where the x+1 came from?

as far as the slope goes ... either it is given or there is another given point on the line so that it may be calculated.