Given that the density of heptane is

The mass of heptane is

The density of water is

The mass of water is

The volume of heptane will be

The volume of water will be

Thus, the volume of heptane is 45.32 mL and the volume of water is 37 mL.
The total volume of liquid in the cylinder will be

The total volume of liquid in the cylinder will be 82.32 mL.
So the initial velocity is 15 m/s, the final velocity is 0 since it's at a complete stop and time is 10 seconds. Therefore:

Therefore, the acceleration is -1.5 m/s^2. The reason it's negative is due to the fact that the vector is going against it's original movement since it's decelerating.
It means that you consider the elements as a list organized by atomic number, the property is seen to repeat over and over as you move through that list.