We know the, isosceles triangle contains two equal sides.
From the question it is given that, one of the two equal sides is 18cm.
And Perimeter of an isosceles triangle is 50cm
We know that, perimeter of isosceles triangle = sum of all sides
Let us assume the third side be x.
Then,
50 = 18 + 18 + x
50 = 36 + x
x = 50 – 36
x = 14 cm
Therefore, length of third side of isosceles triangle is 14 cm.