If f '(x0) = 2, find Lim [f (x0-k) - f (x0)] / 2K, K tends to 0

If f '(x0) = 2, find Lim [f (x0-k) - f (x0)] / 2K, K tends to 0

Let m = x0-k
be
im[f(x0-k)-f(x0)]/2k
=im[f(m)-f(m+k)]/2k
=-im[f(m+k)-f(m)]/2k
=-f'(m)/2
Because M = x0-k
So when k goes to zero
f(x0)=f(m)
So the original formula = - 2 / 2 = - 1