Contextual Applications of Differentiation · Easy problem

The number of visitors to a website on day $t$ is $f(t)$ thousand. Given $f(5)=12$, a marketer uses the tangent-line approximation $f(5.2)\approx f(5)+f'(5)(0.2)$ and finds $f(5.2)\approx12.9$. If this growth rate continued unchanged, predict $f(6)$, the number of thousands of visitors on day $6$.