How do you number 3 and can you explain everything for detail

-y = x + 3   . . . . this is the sides for the double lines

2y + 24 = x + 9 . . . . this is the sides for the single lines

To solve you need to substitute one equation into the other

-y = x + 3
y = -x - 3  . . . place this equation into the other equation where (Y) is

2y + 24 = x + 9
2 ( -x - 3) + 4 = x +9
-2x - 6 + 24 = x + 9
-2x +18= x+9
-3x = -9
x = 3  . . . substitute this answer into the equation and solve for (Y)

-y = x + 3
- y = 3 +3
-y = 3 + 3
 y= -6