At a depth of x- feet
$$Concentration=3+\frac{4}{\sqrt{5-x}}\ parts\ per\ millions$$
At what depth (feet) is the concentration = 6 parts per million
$$6-3=\frac{4}{\sqrt{5-x}}$$
$$3=\frac{4}{\sqrt{5-x}}$$
$$\frac{\left(3.\sqrt{5-x}\right)}{3}=\frac{4}{3}$$
$$\left(\sqrt{5-x}\right)^2=\left(\frac{4}{3}\right)^2$$
$$5-x=\frac{4^2}{3^2}$$
$$5-x=\frac{16}{9}$$
$$-x=\frac{16}{9}-5$$
$$-x=\frac{16}{9}-\frac{5}{1}$$
$$-x=\frac{16-45}{9}=-\frac{29}{9}$$
$$x=3.2$$