AMO 2021 - Secondary 4 (Grade 10) Question 16

Let $f(x)=x^2-4x+5$, with $A=(5,f(5))$ and $B=(5+w,f(5+w))$ on its graph. As $w$ approaches zero through nonzero values, what value does $r=\dfrac{f(5+w)-f(5)}{w}$ approach?