Add preliminary solution for 3c

exam/ex.tex

@@ -166,7 +166,10 @@
       to the total number of evaluations needed, which is $2230$ evaluations of
       $P$.
 
-    \item \TODO
+    \item In order for $(x,y)$ to be a valid preimage for the compression
+      function $F^P$, $x$ must be of size $\unit[655]{bits}$ ($=e$ in the
+      diagram) and $y$ must be of size $\unit[101]{bits}$. Furthermore, the
+      following condition must be true: $[F^P(x)=y]\vee [F^P(y)=x]$.
 
     \item \TODO