hyp = 5cm, adj =  \(\frac{8cm}{2}\) 4cm, opp = xcm

Pythagoras theorem: 

hyp\(^2\) = opp\(^2\) + adj\(^2\)

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

x\(^2\) = 25 - 16

x = √9

x = 3cm