Answer:

Let us first understand what is isoceles triangle :-

Isoceles triangle :- A triangle in which two sides are equal is called an isoceles triangle.

Now, here we are given that two sides are 3 cm and 7 cm and it is an isoceles triangle.

So, the third side should be either 3 or 7.

If the third side is 3, then the sides of the triangle will be 3 , 3 and 7.

Here, if we observe that sum of two sides i.e. 3 and 3 is less than the third side 7.

So, this triangle is not possible.

Now, if we take the third side as 7, then the sides of triangle will be 3 , 7 , 7.

Here, if we take sum of any two sides, it will be greater than the third side.

So, this triangle is possible.

Length of third side is 7 cm.