Brain growth and iqs the researchers at the institute also

Question

Question: BRAIN GROWTH AND IQS. The researchers at the Institute also measured the thickness (also in millimeters) of the cortex of children of age t yr who were of average intelligence. These data lead to the model

A(t) = -0.00005t³ - 0.000826t² + 0.0153t + 4.55           (5 ≤ t ≤ 19)

Show that the cortex of children with average intelligence reaches maximum thickness at age 6 yr.