The function f is defined by f ( x ) = 8 x . For what

Choice B is correct. Substituting $72$ for $f\left(x\right)$ in the given function yields $72=8x$. Dividing each side of this equation by $8$ yields $9=x$. Therefore, $f\left(x\right)=72$ when the value of $x$ is $9$.

Choice A is incorrect. This is the value of $x$ for which $f\left(x\right)=64$, not $f\left(x\right)=72$.

Choice C is incorrect. This is the value of $x$ for which $f\left(x\right)=512$, not $f\left(x\right)=72$.

Choice D is incorrect. This is the value of $x$ for which $f\left(x\right)=640$, not $f\left(x\right)=72$.