The third one, the slope is -1 you recalculate it using the gradient rule= y^2-y^1/x^2-x^1 , choose any two points and one point is one and the other is 2 foe example :
x y
(0,3)1
(3,0)2
-3/3 = -1
and the line intercept the y axis at 3 so the y-intercept or the C =3