If you are searching for the number that makes the whole expression a perfect square, you need to take b, divide it by 2, then square it, like this:

In this case, b is -7. You would then divide it by 2 to get 3.5, then square it.

You can plug this into a calculator to get an exact value, but this is what c is supposed to be to make a perfect square trinomial. 'b' is always the number next to 'X'.