Oooooh Maths thread, why didn't i see this earlier?? Ok Spine, here goes You have y - 3x^2 Slope of a tangent = dy = 6x dx Therefore, slope at the given point = 6(a) = 6a Equation of a line can be written as y = mx + c We know that m = slope = 6a So y = 6ax + c In order to find the value of c, we use the fact that the point (a,3a^2) lies on the line. So substituting the point into the line equation, 3a^2 = 6a^2 + c which gives us c = -3a^2 So the required equation is y = 3ax - 3a^2 As for the second part, we just substitute the point (0,-12) into the above line equation. So, -12 = 3a(0) -3a^2 So we get a^2 = 4 So a = 2 or -2 And that's about it. I hope the solution is clear enough. Edit : Hiss is super ninja