The velocity of a ball left in the wind is described by the expression v_x = 3ct² m/s; time is measured in seconds, c is a constant. At t₀ = 0 s, the ball has an initial position x₀ = - 8.2 m. At a later time t₁ = 4.1 s, the ball is located at x₁ = 8.2 m. What is the value of the constant expressed with the right units?