how to solve?

[gabriel]

The concentration of a certain chemical in a full water tank depends on the depth of the water. At a depth that is x feek below the top of the tank, the voncentration is 3 + (4 / sqrt(5 - x)) parts per million, where 0<x<4. To the nearest 0.1 foot, at what depth is the concentration equal to 6 parts per million?

a. 2.4 ft
b. 2.5 ft
c. 2.8 ft
d. 3.0 ft
e. 3.2 ft

The OA is 3.2 ft...but how?

3 + (4/sqrt(5-x)) = 6 ... 4/sqrt(5-x) = 3 ... squaring both the sides .... 16/(5-x)= 9 .. 16/9 = 5-x .. x = 5 - (16/9) .. that should be approximately 3.2 ..