Volume of a cylinder = the area of the base x height of the cylinder
Area of base =

r^2
A = 6^2(

)
A = 36

V = 36

x 12
V = 432

5 cylinders = 5V
5V = 5 x (432

)
5V = 2160

Final answer: Alex needs to buy 2160

or 6785.84 cubic feet worth of compressed air to fill all of the cylindrical tanks.