Find the critical value to f

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While reviewing a document online, Jordan's cursor moves up and down. For times t > 0, its height above the bottom of the screen is given by the function h(t) = t – 4t³ + 4t? On what time interval is the cursor moving down?

Determine all critical points for the function. y =x° + 3x - 189x +7 =x* + The critical points are at x = (Use a comma to separate answers as needed.)