Math CentralQuandaries & Queries


Question from victoria, a student:

i need help trying to make x the subject in this equation:
7y + 2x = 15
i'm really stuck so any help will be good,



I am going to illustrate with a similar equation, 3x - 5y = 12.

The task is to manipulate the equation, always maintaining the equality and end with an equation of the form

x = something.

The allowable operations that will maintain the equality are adding the same amount to each side of the equation and multiplying each side of the equation by the same amount.

In my equation 3x - 5y = 12 I don't want the -5y term on the left of the equality so I add 5y to each side to obtain

3x - 5y = 12
3x - 5y + 5y = 12 + 5y


3x = 12 + 5y

This is almost what I want, only the 3 on the left side is a problem. If I divide each side of the equation by 3 I get

3x = 12 + 5y
3x/3 = (12 + 5y)/3


x = 12/3 + 5y/3

which is

x = 4 + 5y/3.

Now try your equation,

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