Question

Two side of a traingle are 15cm and 12cm the perimeter of the traingle is 54cm length of the third side

Answer 1

let the third side be x
So,
15+12+x=54
27x=54
x=54-27
x=27
Thus,
the third side is 27cm

Answer 2

Perimeter of triangle = Sum of all sides
54 = 15 + 12 + x
54 = 27 + x
54 - 27 = x
27 = x