-- If a velocity/time graph is a straight line, that tells us that the acceleration is constant.
-- If the line has negative slope, then we know that the acceleration is negative.
. . . As long as the line is above the x-axis, the object is moving forward but slowing down.
. . . When the line drops below the x-axis, the object is moving backward and speeding up.
(If you meant that the line has negative-something-else, then I don't know what to tell you about that part.)
