The position of a particle moving along the x axis is given by zx=12+10t-3.0t^2 m , where t is in s. What is the average velocity during the time interval t = 1.0s to t = 3.0s?

Is the answer -6.0m/s or -4.0m/s or -2.0m/s?

I dofferentiate x to grt dx/dt = 10-6t , substitue the times, answer is 4 & -8. Total disppacement is -8-4 = -12. Time elapsed is 3 s? So -12/3 = -4m/s. Is this correct?

Sorry, not right - it's actually easier than this. The average velocity = change in position divided by time. So determine the value of x(3) - x(1), and divide by time to get average velocity.