So first, you get the absolute value to one side:

Next, set up two equations; One where the value inside the absolute value lines is positive, and another where it is negative, and solve both for the variable:

Your answers are
8 and -8, or +-8.
Answer:
About 5,000 kilograms
Step-by-step explanation:
Look on the x-axis for 18 years. Then go up to where the plotted line is on 18 years to find the y-axis or the mass.