Answer
$f(x) = 0.4(x-4)^2+3.2$
Work Step by Step
Start by graphing the data and the functions in the same figure window.
$$\begin{aligned}
f(x) &= 0.4(x-4)^2+1.2 \ ,\ k= 1.2\\
f(x) &= 0.4(x-4)^2+2.2 \ ,\ k= 2.2\\
f(x) &= 0.4(x-4)^2+3.2 \ ,\ k= 3.2\\
f(x) &= 0.4(x-4)^2+4.2 \ ,\ k= 1.2\\
\end{aligned}$$ The graph shows that the parabola that best fit the data is $$f(x) = 0.4(x-4)^2+3.2 .$$
