The derivative f'(a) is the slope of the line that just grazes the curve at x = a. If that tangent is drawn, its slope is the answer — and for y = x² + c you can also compute 2a, since a constant vertical shift drops out when you differentiate.

Steps

1. Identify the point of tangency T and its x-coordinate a.
2. Either read the steepness of the red tangent or compute f'(a) = 2a.