just integrate the function 6sin(x). you get 6 cosine(x). Y = mx +b where your equation is y = 6 cos (x) and your points are pi/6,3.
I think you mean just take the derivative. The integral of 6sin(x) is -6cos(x). But you put the right answer, 6cos(x) so I assume you just used the wrong word. Right math, wrong word =)