f(x)= x^2 -2x +3

a tangent at x=2 is y=2x=1

The tangent line is used to approximate values of f(x). A satisfactory approximation occurs if the resulting error is at most 0.4

a) Find the error when x=2.5

I won't bother showing my working for this question but my answer was 0.25

b) Show that the greatest value of x which can be used for this approximation is

(10+√10)/5

For this part though, I am just not sure what to do. If someone could show some working out or give some assistance, it would be greatly appreciated