Consider the curve defined by x2 + y2 − 7x + 13y = 12. (a) Find dy dx . dy dx = (b) Under what condition on x is the tangent line to the curve horizontal? The curve has a horizontal tangent line when dy dx is ---Select--- , which occurs when x = . (c) Under what condition on y is the tangent line to the curve vertical? The curve has a vertical tangent line when dy dx is ---Select--- , which occurs when y = .