Mark M. answered 10/08/19
Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
f'(c) = limx→c [(f(x) - f(c)) / (x-c)]
So, if we let c = 3 and f(c) = f(3) = -6, then the given limit is equal to f'(3).
Tangent line to the graph of y = f(x) at the point (3,-6) has slope -10.
Equation of tangent line: y - (-6) = -10(x - 3)
y + 10 = -10x + 30
y = -10x + 20