With simultaneous equations and multiple unknowns, you need several pieces of information. Generally you need one equation for each unknown quantity. So if you have two unknowns you need two equations.
And you can't "cheat" by rearranging the first equation and using that as the second, because when you try to solve it you will end up getting an equation like
or
that is as useless as it is true.
For example the equation
has infinitely many possible solutions for
and
, so you need more information to go off.
Suppose you are then given
.
So now you have two equations:
We can rearrange the second equation, giving us
and then we can substitute this back into the first equation, giving:
And from there it is possible to solve for
. Once you know
you can then solve for
.
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In some more common cases it may be better to add the equations together:
Or subtract them:
But ultimately what you're trying to do is to focus on one variable by creating an equation without all the others. Once you have that variable you can use it to find the other(s).