The essentials
An equation states that two expressions are equal. To keep an equation balanced, apply the same operation to both sides.
For an equation of the form ax + b = c, first subtract b from both sides, then divide by a, provided a is not zero.
Worked example
Solve 3x + 6 = 21.
Subtract 6: 3x = 15.
Divide by 3: x = 5.
Check: 3 × 5 + 6 = 21.