Factor out it by 2.
2(b² - 13b + 40) = 0
b² - 13b + 40 =0
This equation can be solved using the quadratic formula. If you know what a quadratic formula is, then you can get the two values for b. For instance,
(b - 8)(b - 5) = 0
Separate the two "roots", you will get:
b - 8 = 0
b = 8
and
b - 5 = 0
b = 5
Therefore the two values of b are 8 and 5.