I >The submarine is at the surface of the ocean → this is the starting point, if you draw the depth in the y-axis and time in the x-axis it will represent: time zero and depth zero (0,0)
II >Then it descends at a constant rate up until 300feet → given that the speed is a constant, it means that the depth varies the same number of feet for each time unit. This segment of the travel can be represented as a decreasing line (with negative slope)
III >Once at 300 ft depth the subtarine travels for a while without changing it, this segment can be represented by an horizontal line.
IV >The subtarine starts descending again at a constat speed until a depth of 600 feet, as before, this can be represented by a decreasing line (with negative slope)
V >Then it travles some time at this depth, you have to represent this with a flat line
VI >Finally the submarine travels to the surface at constant speed, in this segment of the travel, the variation of y per unit of time is positive, so you have to draw a increasing line (with slope positive) until the x-axis.