Solution

πsid (δage > 20 Sailors) − πsid ((δcolor =  red Boats) ⋈ Reserves ⋈ Sailors)

1. We are dividing the entire query into two parts separated by '−' sign.
2. In the first part, we will fetch a list of all the sailors who are of age greater than 20, that is, age > 20 by using 'δage > 20 Sailors'.
3. In the second part, we will fetch those boats which are of 'red' colour by using 'δcolor =  red Boats'.
4. Then we will join the result with Reserves and Sailors by '(δcolor =  red Boats) ⋈ Reserves ⋈ Sailors'.
5. Finally, we will select the Sid(s) from the 4th step.
6. Now, we will subtract the list of sid(s) we got from step 5 from step 2.