Answer:
9.2cm
Step-by-step explanation:
Let the sides be a, b and c
Let the unknown side be c
a = 6.9cm
b = 11.5cm
c =?
P = 27.6cm
P = a + b + c
27.6 = 6.9 + 11.5 + c
27.6 = 18.4 + c
Collect like terms
27.6 — 18.4 = c
c = 9.2
Therefore, the length of the third side is 9.2cm