Final answer:
To solve the equation b + 21 = 3b + 6, subtract b from both sides, subtract 6 from both sides, divide by 2 to solve for b.
Step-by-step explanation:
To solve the equation with variables on both sides, we need to isolate the variable on one side of the equation. In this case, we have the equation b + 21 = 3b + 6. To isolate the variable, we can subtract b from both sides, which gives us 21 = 2b + 6. Next, we can subtract 6 from both sides to get 15 = 2b. Finally, we divide both sides by 2 to solve for b, giving us b = 7.5.