Undoing Subtraction
Part of the Fundamentals section of Coddy's Algebra journey. Lesson 7 of 64.
In x - 4 = 9 the unknown has 4 subtracted from it. To undo the subtraction, add 4 to both sides:
x - 4 + 4 = 9 + 4
x = 13
Check by substituting 13 into the original equation: 13 - 4 = 9 is true, so 13 is the solution.
After solving, substitute your answer into the original equation and compare the two sides. If the sides are not equal, the answer is not a solution. This check catches calculation mistakes. In later lessons it also catches extra answers that some steps can introduce.
Challenge
EasySolve the equation x - 6 = 15 for x.
Try it yourself
Solve for x
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Fundamentals
1What an Equation Is
The EquationFinding the UnknownThe Same to Both SidesSolved Means AloneRecap: First Equations4Two-Step Equations
Two Layers to UndoSubtract, Then DivideAdd, Then DivideA Division InsideNegatives in Two StepsRecap: Two-Step Equations2One-Step Equations
Undoing AdditionUndoing SubtractionUndoing MultiplicationUndoing DivisionInverse OperationsRecap: One-Step Equations5Tidying First
Like TermsTerms and Numbers MixedThe Distributive PropertyDistributing a SubtractionTidy, Then SolveRecap: Tidying First