Point-slope form is one way of describing a straight line. It is useful when you have a line to be described and you know the coordinates of one point on the line and its gradient (slope).
The point-slope form equation of a straight line for a slope, m, and known coordinate [tex](x_1, y_1)[/tex] is [tex]y-y_1=m(x-x_1 )[/tex].
So for your question you have [tex](x_1, y_1) = (4, -9)[/tex] and slope = m = -1. You just put these numbers into the formula [tex]y-(-9)=-1(x-4)[/tex] and this is your answer: the equation of the line.
