A disk lies in the xy plane and rotates about its central axis that is parallel to the z-axis.  At t = 0, the angular velocity of the disk, ω_z = -4.80 rad/s. At t = 7.00 s, the angular velocity has increased steadily to +8.90 rad/s. Taking counterclockwise rotation as positive, determine magnitude and state whether the angular acceleration is positive or negative during this time.