please help with questions 12-14 :)

. Question 12 0/1 pt @ Details 8 sin x Let f (a) = 2 sin x + 6 cos x Then f'(x) = Round your answers to 3 decimal places. The equation of the tangent line to y = f (x) at a = can be written in the form y = ma + b where m and b = Question Help: G VIDEO B Written Example Message instructor D Post to forum Question 13 0/1.pt @ Details Find all a values on the graph of f (x) = 4x - 6 cos x for 0