You need to calculate volume.
The shape of the tank is cylindrical because of it mentioning diameter. Since a radius is half the length of a diameter, we can say that r = 21 inches. H = 6 feet = 72 inches.
Volume of a cylinder: V = π(r^2)h
V = π(21^2)(72)
V = 99751.85 in.^3
If you needed the volume in cubic feet, multiply cubic inches by 0.00058 (source: http://www.boatcycle.com/calcs.php)
V = 57.856 ft.^3
Now, from that same source, it says that 1 cubic foot is equal to 7.5 gallons of water. What we need to do is multiply 57.856 by 7.5, which will give us:
434.670548 gallons of water.
