Ok first you need to find the slope by using the slope formula, y2 - y1 / x2 - x1. After you do that you should have gotten -4/12 with you simplify to -1/3. Next you use subsitution with the order pair (-6,-3) and put it into a linear equation y = mx + b. Once you do that you should of gotten -3 = -1/3(-6) + b, from here use algebraic steps to find out the y - intercept. The y - intecept is -5, and with all the information founded out you put it in slope intercept form to get.
y = -1/3x - 5
Please tell me if I got it wrong or if my explanation isn't sufficent.