A two-step equation is an equation which needs two steps to solve.
In this case, answer choice "C" is a two-step equation.
2x + 9 = 21. Subtract 9 from both sides
2x = 12. Divide both sides by 2.
x = 6.
Answer choice "B" just gives us the answer.
Answer choice "A" needs only one step to solve the equation.
x + 9 = 21. Subtract 9 from both sides.
x = 12.
