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
is 
.
So for your question you have <span>
</span> and slope = m = -1. You just put these numbers into the formula 
and this is your answer: the equation of the line.
The point-slope form equation of a straight line for a slope, m, and known coordinate
So for your question you have <span>