The rule of the volume of the rectangular prism is

L is the length
W is the width
H is the height
Since the tank is 6 2/3 ft wide and 4 2/3 height, then


Since the volume of the tank is 350 cubic feet, then

Substitute them in the rule above

Divide both sides by 280/9 to find L

Make it a mixed number
Since 45/4 = 11 1/4

The length is 11 1/4 feet