SOLUTION: The completion of a certain chemical reaction is expressed by the equation y=250-5x-x^2, where y is the number of seconds needed to complete the reaction and x is the temperature i

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Solve for x:
where y = number of seconds (200) to complete the chemical reaction and x = the temperature, in degrees celcius, at which the reaction occurs. Substitute y = 200
Subtract 200 from both sides.
Multiply through by -1 to ease the process a bit.
Solve the quadratic equation by factoring.
Apply the zero product rule: If a*b = 0, then either a = 0 or b = 0 or both.
or so that...
or
So you have two answers, as you would expect from a quadratic equation, but it is reasonable to expect that chemical reactions are more likely to occur at higher temperatures than at lower temperatures, so I think that the answer would be 5 degrees celsius.