Chapter 3 - Section 3.1 - Quadratic Functions and Models - 3.1 Exercises - Page 253: 56

maximum serum concentration reached at $t=150min$ maximum concentration $C(t)=4.5 mg/L$

Calculate the maximum of the function by substituting the values from the function into $t_{MAX}=\frac{-b}{2a}\rightarrow t_{MAX}=150$ which is the number of minutes. Then substitute $x=150$ into the function to get the maximum concentration.