Q+B=13. When B+3=0
You have 2 separate equations.
First solve the 2nd equation for B:
B +3 = 0
Subtract 3 from both sides:
B +3 - 3 = 0-3
Simplify:
B = -3
Now we have a value for B, we can replace b with -3 in the first equation and solve for Q:
Q + -3 = 13
Add 3 to each side:
Q + -3 + 3 = 13+3
Simplify:
Q = 16
So you have Q = 16 and B = -3