Answer:
the sum of x and 15 is -16 because it is a straight line which means the straight line is a function.
For this case, what you should do is use any computational tool and graph the data in the table. For example, I used excel.
In the x axis we have Starting temperature and in the axis of the y we have Ending temperature.
You can see how the diagram varies almost linearly.
X=Marks sister's age
x+11= Marks age
x+11 +8 = 2x+8
x+19=2x +8
x+19-x=2x-x +8
19-8=x+8-8
11=x Marks sisters age in 8 years
11-8= 3 Marks sisters age now
x+11=22 Marks age in 8 years
22-8= 14 Marks age now