The limit as x approaches -3 of [g(x) - g(-3)] / [x -(-3)] is, by definition of limit, g '(x) at x = -3.
And g '(x) at x = -3 is the slope of the line tangent to it.
2y + 3 = -(2/3) (x-3)
2y = -2x/3 + 2 - 3
y = -x/3 - 1/2 => slope = -1/3
Then the limit is -1/3
Answer: - 1/3