Answer: The value of c must be 36 to make it perfect square trinomial.
Step-by-step explanation:
A standard perfect square trinomial is
where
is the constant term.
Given trinomial

On comparing with standard perfect square trinomial we get a=-6
Therefore the constant term c=

Such that the trinomial becomes
a perfect square trinomial.
Hence the value of c for the given trinomial must be 36 to make it perfect square trinomial.