The amount of nitrogen dioxide, a brown gas that impairs breathing, present in the atmosphere on a certain may day in the city of long beach is approximated by

a(t) = 136/1+ 0.24(t − 4.5)^2 + 28 (0 ≤ t ≤ 11) where a(t) is measured in pollutant standard index (psi) and t is measured in hours, with t = 0 corresponding to 7 a. m. determine the time of day when the pollution is at its highest level.