text: 98.7te^(-.31t)? t=time, find peak concentration put the first derivative equals to zero, and find t  f(t) = 98.7 t exp(-.31 t)  f'(t) = 98.7 t (-.31) exp(-.31 t ) + 98.7 exp(-.31 t)  put f'(t) =0 -->  0 =98.7 t (-.31) exp(-.31 t ) + 98.7 exp(-.31 t)  0 = exp(-.31 t )( -.31 t + 1 )  either exp(-.31 t) =0 ---> illegal or -.31 t + 1 =0 --> t = 3.2258