A container with chemicals breaks, accidently releasing its content into a river flowing

at $v=0\mathrm{.}6k\frac{m}{h}.$ When experts of the environmental protection agency arrive to take

water samples, the maximum concentration of $860ppm$ is located $2km$ downstream

the release point. $50m$ from the maximum, the concentration is $410ppm.$

a$)$ How large is the dispersion coefficient?

b$)$ What will the maximum concentration be $15km$ downstream?