The graph of the function $y=f(x)$ is symmetrical about the line $x=2$ then

A function is symmetrical about a vertical line $x=a$ if for every $x$ in the domain, moving a distance $d$ to the left of $a$ and a distance $d$ to the right of $a$ results in the same function value.
Mathematically, this is expressed as $f(a+x)=f(a-x).$
Given the line of symmetry is $x=2$:
$f(2+x)=f(2-x)$.