Original position of the submarine below the Surface = -2 2/3= -8/3
Submarine at a depth of -2 2/3 feet descended by 5 1/2 feet.
So new position of the Submarine =

To Reach the surface submarine must rise = 0-(-49/6)
As seen from the equation the two negative sign [(-)(-)] will change into a positive sign.
At the surface the position of the submarine = +49/6 feet