you have an awkward way of writing the derivative. notice that

anyway, remember that the derivative gives the slope. since the slope of the line is 1/6, you want the tangent line passing through the point x such that the derivative is 1/6, that is:

solve for x, and you can find the coordinates of the point. then use that point in the point-slope form to find the equation of the line.

remember the point slope form says the equation of a line is given by:

where is the slope of the line and is a point the line passes through