Why does my child rearrange y = 3x + 2 as x = y/3 − 2?
Your child undoes the operations in the wrong order when rearranging a formula, such as y = 3x + 2 giving x = y/3 - 2.
Why children do it
To make x the subject, what was done to x is undone in reverse order. In y = 3x + 2, x was multiplied by 3, then had 2 added, so the 2 comes off first: y − 2 = 3x, then x = (y − 2)/3. Undoing in the order things were done gives a different formula.
How to help at home
Ask them what happens to x first in the formula, and what happens last: undo the last thing first.
The right way
Make x the subject: y = 3x + 2
- y − 2 = 3xSubtract 2 from both sides.
- x = (y − 2)/3Divide both sides by 3.
Find out where your child is
A free maths check, in about 15 minutes, shows what your child knows and the mistakes they make. No sign-up.
Start the free checkRelated mistakes
- Why does my child solve x − 4 = 10 as x = 6?
- Why does my child solve x + 5 = 12 as x = 17?
- Why does my child solve 3x = 12 as x = 36?
- Why does my child solve x/3 = 5 as x = 5/3?
- Why does my child solve x/3 + 4 = 9 as x = 27?
- Why does my child change only one side of an equation?
- Why does my child think 2x + 4 = 10 becomes 2x = 14?
- Why does my child think 5x + 3 = 2x + 15 gives 7x?
- Why does my child solve 4(x − 2) = 12 as x = 14?
- Why does my child get the method right but the answer wrong?
- Why does my child swap the values of x and y?
- Why does my child mix up the gradient and the y-intercept?
- Why does my child solve −2x > 6 as x > −3?
- Why does my child draw y = 2x − 1 through (−1, 0)?
- Why does my child multiply only the fractions in an equation?
- Why does my child use ≥ for a dashed line?
- Why does my child write y < 2x − 1 for the region above the line?