4. A chemical cleaner from a factory is accidentally spilled into a nearby lake. The concentration, c, in grams per litre of cleaner in the water t days after it is spilled can be modelled by the function c(O)= 120 2t2+9 a) At what rate is the concentration of the chemical cleaner changing one week after the spill? b) For what values of t is the instantaneous rate of change for this function equal to 0? [6]