Estimate how much the function f (x) = (x / x +1 ) -3 will change as x decreases from 4 to 3.8

If anyone could explain this, that would be great..practice problem for a test coming up.

I got -0.008 would like to check it.

Also have another question,

find the derivative f(x) = 2x + 5 / (1 - 2x)^3