Answer
$230nm$
Work Step by Step
2 times the thickness of the coating, $x$, should equal a whole number of wavelengths for it to be the brightest light reflected. $2x=m\lambda_c$. Assuming $\lambda_c=\frac{\lambda}{n}$, $x=\frac{m\lambda}{2n}=\frac{(570nm)}{2(1.25)}=230nm$
