I assume that is supposed to be x squared. A perfect trinomial is the product of a perfect square, (x times a number) squared. With these trinomials, you can look at them as: ax squared times bx + c. When a is 1, the fastest way to get c to make a perfect square is to divide b by 2. In this case, it's 4/2 = 2. The last step is to square that number. 2 squared = 4.
