well, standard form would be y-y1= m(x-x1). so, once you find the slope, just plug the respective values into that equation.
m= 6+3 over -6-3 = 9/-9 = -1
now, just choose one of the points given to you in the problem and plug it into the equation.
y-6 = -1(x+6)
or,
y+3= -1(x-3)