Find the value of df-1/dx at x = f(a).

Question

Find the value of df^-1/dx at x = f(a).

f(x) = 2x + 7, a = -1

MULTIPLE CHOICE

1/2

1/7

2

7

Answer 1

f(x)=2x+7

find f^-1(x)

x=2*f^-1(x)+7

f^-1(x)=(1/2)x-7/2

d(f^-1(x))/dx=1/2

I am confused with x=f(a) , a=-1

BUT

since the derivative is constant for all x, answer is 1/2