I presume you know that the slope of the tangent line to a curve, at a specific point, is the derivative of the function at that point. The derivative here is f'(x)= -6sin(3x). On the interval from 0 to , -sin(x) has minimum value of -1 at and so the minimum slope of a tangent line to this curve is -6 at and . Use either one of those to findanequation of a tangent line.