Answer:
is to be added to make
a perfect square trinomial as 
Step-by-step explanation:
Given :
We have to find what can be added to
to form a perfect square trinomial.
Perfect square trinomial is of the form 
Consider the given expression
Using algebraic identity
,
Comparing , we get, a = x ......(1)

Uisng (1) , we get,

To make it a perfect square trinomial we need to add
term

Thus,
is to be added to make
a perfect square trinomial as 