Start by finding the slope using m = (y2 - y1)/(x2 - x1). Substituting in you get (-1 + 3)/(6 - 3). So m = 2/3
Next, use y - y1 = m(x - x1) and substitute any of the coordinates in: y + 3 = 2/3(x - 3). That gives you y + 3 = (2/3)x - 2. So tha final answer is y = (2/3)x -5