We want to find the unknown value b in the following equation
-11b + 7 = 40
Since both sides are the same we can substract 7 both sides and it will be true
-11b + 7 - 7 = 40 - 7
-11b + 0 = 33
-11b = 33
We want to have just b in one side of the equation, we can divide both sides by -11, since they are equal:
-11b = 33
[tex]\begin{gathered} \frac{-11b}{-11}=\frac{33}{-11} \\ 1\cdot b=-3 \\ b=-3 \end{gathered}[/tex]