When we solve algebraic equations, our aim is to end up with one letter on one side of the equals sign and one number on the other. This is the solution.
We do this by simplifying like terms if we can and then using inverse operations to undo things that get in the way, but remember that we must do the same thing to both sides.
7b - 2b = 20
Simplify the left hand side by combining like terms
5b = 20
Divide both sides by 5
5b ÷ 5 = 20 ÷ 5
b = 4