Internal Standard Methodology
However my question is which values need to be used for the $A_\mathrm x$ and $A_\mathrm{IS}$ - do they need to be both morphine or just the two values from the sample peak areas?

To answer just this part of the question,
\begin{align}
A_\mathrm{x} &amp;= A_\mathrm{unknown} \mathrm{(Morphine)} \\
A_\mathrm{IS} &amp;= A_\mathrm{standard} \mathrm{(Codeine)} \\
C_\mathrm{x} &amp;= C_\mathrm{unknown} \mathrm{(Morphine)} \\
C_\mathrm{IS} &amp;= C_\mathrm{standard} \mathrm{(Codeine)} \\
\end{align}
Looking at your picture, it looks like you successfully calculated your $\mathrm {D_{rf}({morphine})}$ (though it's next to a misleading picture, as the peak area of codeine is larger than that of morphine when their concentrations are equal).
Moving down, you've successfully solved your $\mathrm{D_{rf}}$ equation for unknown concentration, but the substitution is a little off. When you're dealing with the dissolved tablet solution, $A_\mathrm{unknown}$ is indeed $17.6$ but $A_\mathrm{standard}$ would be the codeine area of $212.5$, not the earlier morphine value.
If I had to guess, the confusion is between the terms &quot;internal standard,&quot; a known spike of (usually) a single standard (here codeine) and &quot;standard solution,&quot; which is an reference standard of known morphine and internal standard concentrations. Two common uses of reference standards are as initial calibrants and a good way to check for interference between analytes.
Getting back to the solution, once you have your unknown concentration, in units of $\pu{g/L} = \pu{mg/mL}$, you will simply multiply by the $\pu{50 mL} = \pu{0.05 L}$ volume to have the morphine concentration of the solution and thus of the whole $\pu{200mg}$ tablet. %w/w is also easy at that point.