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Derivative of velocity then critical points question (Optimization problem)

__Question:__

(1 pt) A University of Rochester student decided to depart from Earth after his graduation to find work on Mars. Before building a shuttle, he conducted careful calculations. A model for the velocity of the shuttle, from liftoff at t = 0 s until the solid rocket boosters were jettisoned at t = 63.4 s, is given by

(in feet per second). Using this model, estimate

the absolute maximum value

and absolute minimum value

of the ** ACCELERATION ** of the shuttle between liftoff and the jettisoning of the boosters.

__My problem:__ (My work is attached) When I attempt to use the quadratic formula to solve for x on the derivative function of velocity (=acceleration function), the inside of the square root is negative and I therefore cannot take the square root of it. Can someone please help me out?