We can find the volume of any rectangular prism by simply multiplicating its height, length and width.
Also, lets change that 6 1/4 to 6.25, or 25/4 if you like. I’m gonna use 6.25.
If we take volume as “V”, then:
V = L * W * H
V = 3ft * 6.25ft * 14ft
V = 262.5 ft^3 (Its ft cubed because we multiplied by ft three times.)