okay so, first what is the slope of the line? you have stated it is -1.
so now we have a point on the line (x1,y1) = (4, -9), so now we can use:
y-y1 = m(x-x1)
y - (-9) = -1 (x - 4)
y + 9 = -x + 4
y = -x + 4 -9
y = -x -5
Let's check. If x = 4, then
y = - (4) -5 = -9
seems to be correct
