Respuesta :

sitshaoyuedison

Answer:

10.2cm

Step-by-step explanation:

Perimeter is the sum of the lengths of all sides.

1st side: 9cm

2nd side: 7.8cm

Perimeter: 27cm

Perimeter = 1st + 2nd +3rd side of a triangle

27cm = 9cm +7.8cm + 3rd side

3rd side = 27cm - 9cm -7.8cm

              = 10.2cm