$\text{M}\times \text{N}:13\times 14::\text{F}\times \text{R}:?$

The expression directly represents each letter replaced by its exact numerical position index in the standard English alphabet ($\text{M}=13$, $\text{N}=14$).
Since $\text{F}=6$ and $\text{R}=18$, the equivalent expression is $6\times 18$.