To find the value of b, we must first solve the system of equations given:

1) B1 + B5 = 20
2) B2 + B3 = 17

From equation 1, we know that B1 + B5 = 20. We can rewrite this as:

B5 = 20 - B1

Now, we substitute this into equation 2:

B2 + B3 = 17
B2 + B5 - B1 = 17
B2 + (20 - B1) = 17
B2 - B1 = -3

Now, we know that B is equal to the negative of the difference of B1 and B2. Therefore, the value of b would be:

b = -(B1 - B2)
b = -(B2 - B1)
b = 3

Therefore, b is equal to 3.