The viable answer, according to the provided profit equations, is that they make the same profit after three days, while the nonviable response is -4 because the number of days must be a positive value.
Given that the profit equation of the first friend is
P(x) = −x² + 5x + 12 ...(1)
and the profit equation of the second friend is
Q(x) = 6x ...(2)
The system we use to determine when they earn the same profit is provided by:
P(x) = Q(x)
i.e. −x² + 5x + 12 = 6x
i.e. x² + x - 12 = 0
i.e. x² + 4x - 3x - 12 = 0
i.e. x(x + 4) - 3(x + 4) = 0
i.e. (x + 4)(x - 3) = 0
i.e. x = 3, -4
Because the number of days must be positive, the viable answer is that they earn the same profit after three days, while the nonviable answer is -4.
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