In a disastrous first flight, an experimental paper airplane follows the trajectory indicated by these parametric equations

The airplane was flying horizontally at those times when dy/dt = 0 and dx/dt = 0. From the given trajectory we have dy/dt = 3 sin t and dx/dt = 1 - 3 cos t (6) Setting dy/dt = 0 yields the equation 3 sin t = 0, or, more simply, sin t = 0. This equation has four solutions in the time interval 0 ≤ t ≤ 10: t = 0, t = π, t = 2π, t = 3π I want to know how do they came up with these four solutions, I solved but I got only 1 t=0