Use the function f and the given real number a to find (f)'(a). (Hint: See Example 5. If an answer does not exist, enter DNE.) f(x) = x +7x (F-)'(-9) = || - 1, a = -9 Assume that x and y are both differentiable functions of t and find the required values of dy/dt and dx/dt. y = 2 (x - 3x) (a) Find dy/dt when x = 5, given that dx/dt = 2. dy/dt = (b) Find dx/dt when x = 9, given that dy/dt = 4. dx/dt =