What value of c makes the polynomial below a perfect square?

Question

4x^2+28x+c

 

c=

Answer 1

If c = 49, then we can factor out 4 from 4x² + 28x + 49 to get

 

4(x² + 7x + 12.25)

 

And we know what two numbers have a sum of 7, and a product of 12.25, don't we :)

 

Tutors, please feel free to review my explanation.