X(x-4)is linear, quadratic or neither?

neither because A quadratic function is one of the form f(x) = ax² + bx + c, where a, b, and c are numbers with a not equal to zero. The graph of a quadratic function is a curve called a parabola. A parabola intersects its axis of symmetry at a point called the vertex of the parabola. You know that two points determine a line.

arc length = radius * x ( in radians)

6 * pi = 18 * x

divide by 18 on each side

6 pi / 18 = x

pi/3 = x

the angle is pi / 3 radians

or   60 degrees