Several folks have asked "how do I do #4,6 on the practice quiz?", well, here's a hint. You are looking for straight lines, so you need their slope. You get that from the line 9x-y+3=0 (parallel in #4, perpendicular in #6, so use same slope in the first, negative reciprocal in the second). Then you want that slope you just found to be the slope of a tangent line for the curve y=x^3-3x^2-15x+7 - in other words, you want the derivative for that curve to equal the wanted slope. That gives you an equation to solve for x. Solve it and you've got your points on the curve where you want tangent lines. Then carry on as in question 3. Hope this helps, -= rags =-