Помогите решить 3 вопроса

Решите задачу:

$1) f(x)=-4cosx + sinx\\\\ f^{'}(x) = (-4cosx + sinx)^{'} = (-4cosx)^{'} + (sinx)^{'} = 4sinx+cosx\\\\ f^{'}(\frac{\pi}{2}) = 4sin\frac{\pi}{2} + cos\frac{\pi}{2} = 4*1+0=4\\\\ 3) f(x) = x^{3} - 4x^{2}+7\\\\ f^{'}(x) = (x^{3} - 4x^{2}+7)^{'} = f(x) = (x^{3})^{'} - (4x^{2})^{'}+(7)^{'} = \\\\ = 3x^{2} - 8x\\\\ f^{'}(2) = 3*4 - 8*2 = 12-16=-4$