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How do you go about rearranging equations where the required subject appears on both sides? Such as making x the subject of 7x + a = 3x + b.

To rearrange any equation, you must always do the same to both sides.
If you take a number from one side, you must also take it from the other. If you divide a side by one number you must divide the other by the same number (and this applies to the whole of both sides, not just part of the side).

7x + a = 3x + b, make x the subject:
Begin by collecting like terms on each side (so all the terms involving x on one side, and all the other terms on the other side).
1) Take 3x from both sides:
7x + a - 3x = 3x + b - 3x
4x + a = b
2) Take a from both sides:
4x + a - a = b - a
4x = b - a
3) Divide both sides by 4 (as x must be alone):
(4x)/4 = (b - a)/4
x = (b - a)/4 This is our answer

Rachael T. 11 Plus Maths tutor, GCSE Maths tutor, 13 plus  Maths tuto...

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