The sign of speed equals the sign of slope, so the second thing to understand is that slope equals speed.

The first negative sign of the first question refers to the displacement in the fourth second, not the displacement in the fourth second. Because the fourth second speed is negative, the fourth second displacement is negative, but the fourth second displacement is positive.

The second negative sign in the first question is that the object has moved negatively beyond the initial position in the fifth second, so the displacement in the first five seconds is negative.