first you need to substitute the xs and ys into point slope ( y1 - y2 = m(x1 - x2). in this example it would be 3-6= m(0 - (-1)). then you just simplify, so -3 = m(1) -3=m. the slop is -3. because you already know the y-intercept is 3 (0,3), you can substitute everything into slope intercept (y=mx+b). so the answer is y=-3x+3.
I hope this helped, let me know if you are still confused and i can help :)
