3] A polluted river has an initial concentration of a heavy metals of 10?parts/m3. The concentration of the heavy metals will reduce as rain falls into the rain. The concentration C of the pollutant as a function of time [in weeks] is given by dc 3+ oosac = 0. 6(0) = 107 Using Heun's method and a step size of 3.5 weeks, find the concentration ofthe pollutant at 7 weeks. [6 marks]