select PNO from P Where COLOUR='BLUE';
This can be written as: $\pi_{pno}(\sigma_{colour='Blue'}(P))$
Store this in T1.
$\therefore$ $T1\leftarrow\pi_{pno}(\sigma_{colour='Blue'}(P))$
Then
select SNO from SP
where PNOin (select PNO from P
Where COLOUR='BLUE'))
$T2\leftarrow\pi_{sno}(\sigma_{pno=T1}(SP))$
Similary
Select SNAME from S
Where SNOin (select SNO from SP
where PNOin (select PNO from P
Where COLOUR='BLUE'));
$Result\leftarrow\pi_{sname}(\sigma_{sno=T2}(S))$