Bonsoir,
[tex]1)\ f(t)=200e^{-\dfrac{t}{10}}\\\\f'(t)=200\times(-\dfrac{t}{10})'e^{-\dfrac{t}{10}}\\\\f'(t)=200\times(-\dfrac{1}{10})e^{-\dfrac{t}{10}}\\\\\boxed{f'(t)=-20e^{-\dfrac{t}{10}}}[/tex]
[tex]2)\ f(t)=200\\\\200e^{-\dfrac{t}{10}}=200\\\\e^{-\dfrac{t}{10}}=\dfrac{200}{200}\\\\e^{-\dfrac{t}{10}}=1\\\\e^{-\dfrac{t}{10}}=e^0\\\\[/tex]
[tex]-\dfrac{t}{10}=0\\\\\boxed{t=0}[/tex]