What value of b makes the trinomial below a perfect square x^2-bx+100

1. Perfect square trinomials, are 2nd degree polynomials, of the form           so that , which can be written as perfect squares.

2. For example 

3. Thus are perfect square trinomials.

4. 

5. In the first case -b=20, so b=-20. In the second case, -b=-20, so b=20.

6. b∈{-20, 20}

Ican’t answer if there is no diagram