Speed is the derivative of distance, x(t), with respect to time.
speed = dx/dt = 4t³ - 30t² + 58t - 36
The time at which the speed is greatest is the derivative of the speed equated to zero.
ds/dt = 12t² - 60t + 58 = 0
The value of t from the equation are 3.69 and 1.31 seconds.