Using Pythagora's rule for △PSM

Hyp\(^2\) = Opp\(^2\) + Adj\(^2\)

5\(^2\) = Opp\(^2\) + 4\(^2\)

→ Opp\(^2\) = 25 - 16

→ Opp = √9 = 3

base SR = 18 + 3 = 21

base PQ = 18

side PS = 5

side QR = 4

perimeter of the trapezium PQRS = 21 + 18 + 5 + 4 = 48cm