Calculus question

Question

The concentration, C in milligrams per litre, of a tracer chemical in the bloodstream t hours after its injection, is given by the formula Ct = e-t-e-3t. Determine the maximum concentration of the chemical. When does this occur?

Answer 1

I don't understand the formula. It looks like C^{t =} e^-t-e^-3t

Derivative is -e^-t + 3e^-3t = 0

e^t = e^3t/3

1 = e^2t/3

e^2t = 3

ln(e^2t ) = ln(3)

2t = ln(3)

t = ln(3) / 2