What value of c makes the trinomial below a perfect square x2 + 7x + c?

We have that
x² + 7x + c
Group terms that contain the same variable
(x² + 7x )+ c
Complete the square. Remember to balance the equation 
(x² + 7x+3.5² )+ c-3.5²

Rewrite as perfect squares

(x+3.5)²+ c-3.5²
so
c-3.5² must be zero
c-3.5²=0 c=3.5²> c=12.25

the answer is
the value of c must be 12.25