Question 27461
Let amount her sister earns be "x"
The samantha earns "x+7" dollars.
EQUATION:
y=x+7
Plot two points:
If x=0 y=0+7=7 gives the point (0,7)
If x=5 y=5+7=12 gives the point (5,12)
Plot  those and draw a line through them carefully.
Now see what the "y" value would be if x were 2.5.
You can check your estimated answer by putting 2.5
into the equation in place of x:
y=2.5+7=9.5
Cheers,
Stan H.