If you differentiate the equation, you get dy/dx = 9cos(9x) - 2sin(2x)
after you have done this, you can put the x value into that to get the gradiant of the curve at the point given, which will be about -1.7
now that you know the gradient of that line, you can use the straight line formula to find out that line, which is (y-Yvalue) = gradient(x-Xvalue) which comes out as y- pi/6 = -1.7(x-pi/6), if you need to find this in the form y=mx+c you can just rearrange so that everything is on the right hand side, hope this helped
