When one equation already says y = ... (or x = ...), put that expression into the other equation.
What you will learn
- Substitute an expression for one letter
- Rearrange first when needed
- Handle answers that are fractions
Replace the letter
- y = 2x − 3
- 3x + 2y = 15
- replace y: 3x + 2(2x − 3) = 15
- 7x − 6 = 15, so x = 3
- y = 2 × 3 − 3 = 3
Brackets matter: 2(2x − 3), not 2 × 2x − 3.
If neither equation has a letter on its own, rearrange the easier one first: x + 3y = 11 gives x = 11 − 3y.
The rest of this lesson and its 7 practice questions are locked.
In this lesson
- Replace the letter
- Two expressions for y
7 practice questions, marked as you go; about 20 minutes in all.
In this unit: Simultaneous equations
Solve two equations at once by elimination, substitution and graphs, form them from problems, and (Higher) solve a linear and a quadratic equation together.