I don't know what the symbol means or what to do.

Answer:
Step-by-step explanation:
That weird symbol means "element". In our case, the elements of the domain (the x-values) are -15, -6, -1. Which tells us that we are going to sub each one of the numbers into the equation and solve it for the corresponding y value. That in turn gives us the coordinate point. The equation is
x + y = 6
Subbing in first -15 for x:
-15 + y = 6 and
y = 6 + 15 so
y = 21 and the coordinate point is (-15, 21).
Subbing in -6 for x:
-6 + y = 6 and
y = 6 + 6 so
y = 12 and the coordinate point is (-6, 12).
Subbing in -1 for x:
-1 + y = 6 and
y = 6 + 1 so
y = 7 and the coordinate point is (-1, 7)
The second choice is the one you want.
Just remember that x is the domain and any elements in the domain go in for x.
Also remember that y is the range and any elements in the range go in for y