I've been given the inverse square law
$$F= L / (4 π d^2)$$
and told to compare the brightness of two stars by taking the ratio of the inverse square law for Star 1 (with quantities $F_1, L_1$, and $d_1$) and Star 2 (with quantities $F_2, L_2$, and $d_2$).
What does it mean to take the ratio of the law for two different values?
Obtain an expressions for either $\frac{{{F_2}}}{{{F_1}}}$ or $\frac{{{F_1}}}{{{F_2}}}$. The ratio expresses the relative magnitude of stellar brightness. Usually choose the first if ${F_2} > {F_1}$. Forming the ratio eliminates the constant $4\pi $.