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Solution

We need sailors whose age is over $20$ and who have not reserved a red boat.

First find sailors with age over $20$.

From Sailors table:

$\pi_{sid}(\sigma_{age>20}(Sailors))=\{22,29,31,32,58,64,74,85,95\}$

Now find red boats.

From Boats table, red boats are:

$Bid=102$ and $Bid=104$

Now find sailors who reserved red boats.

From Reserves table:

$Sid=22$ reserved $Bid=102$ and $Bid=104$

$Sid=31$ reserved $Bid=102$ and $Bid=104$

$Sid=64$ reserved $Bid=102$

So,

$\pi_{sid}((\sigma_{color='red'}(Boats))\bowtie Reserves\bowtie Sailors)=\{22,31,64\}$

Now subtract sailors who reserved red boats from sailors whose age is over $20$.

$\{22,29,31,32,58,64,74,85,95\}-\{22,31,64\}$

$=\{29,32,58,74,85,95\}$

So, the correct relational algebra expression must be:

$\pi_{sid}(\sigma_{age>20}(Sailors))-\pi_{sid}((\sigma_{color='red'}(Boats))\bowtie Reserves\bowtie Sailors)$