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**Macleef** The function $\displaystyle C(t) = e^{-2t} - e^{-5t}, t \geq 0$ can be used to model the concentration at the time $\displaystyle t$ (where $\displaystyle t$ is measured in hours), of a drug injected into the bloodstream. Determine when the concentration level in the body is a max.

I'm not sure if I took the 1st derivative of the polynomial correctly?

$\displaystyle C'(t) = e^{-2t}(-2) - e^{-5t}(-5)$

$\displaystyle C'(t) = -2e^{-2t} + 5e^{-5t}$

$\displaystyle 0 = -2e^{-2t} + 5e^{-5t}$

$\displaystyle 0 = \frac {-2}{e^{2t}} + \frac {5}{e^{5t}}$

$\displaystyle 0 = \frac {-2e^{5t} + 5e^{2t}}{e^{10t}}$

$\displaystyle 0 = -2e^{5t} + 5e^{2t}$

$\displaystyle 0 = -2e^{5t} + 5e^{2t}$

If I took the derivative correctly, I don't know how to simplify it further