Which steps are needed to solve this equation? . b + 6 = 21 Subtract 6 from both sides of the equation. The answer is b = 15. Check the solution by substituting 15 for b. Subtract 6 from the left side and add 6 to the right side of the equation. The answer is b = 27. Check the solution by substituting 27 for b. Add 6 to both sides of the equation. The answer is b = 27. Check the solution by substituting 27 for b. Add 6 to the left side and subtract 6 from the right side of the equation. The answer is b = 15. Check the solution by substituting 15 for b.