To ‘solve an equation for a particular variable’ means
to rewrite the equation with the particular
variable all by itself on one side.
The variable you are solving for cannot appear on the other side of the equation.
Conventionally, the variable being solved for is put on the lefthand side of the equation.
EXAMPLES:
Solving the equation
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$\quad PV = nRT\quad$ for $\quad P\quad$
gives
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$\displaystyle\quad P = \frac{nRT}{V}\quad$.
Solving the equation
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$\quad PV = nRT\quad$ for $\quad T\quad$
gives
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$\displaystyle\quad T = \frac{PV}{nR}\quad$.
The key to solving for a particular variable is to think about
what is being done to the variable,
and then to ‘undo’ these operations, in reverse order.
Question:
Solve the equation
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$\quad ax + b = c\quad$
for
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$\quad x\,$.
Solution:
Here is the correct thought process:
 There is only one
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$\,x\,$
in the equation, in the expression
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$\,ax + b\,$.
 The expression
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$\,ax + b\,$
represents a sequence of operations acting on
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$\,x\,$:
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$$
x \qquad\overset{\text{multiply by $a$}}{\rightarrow}\qquad
ax \qquad\overset{\text{add $b$}}{\rightarrow}\qquad
ax + b
$$
 We must ‘undo’ these operations, in reverse order,
to get back to
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$\,x\,$.
Note that addition is undone with subtraction, and multiplication is undone with division:
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$$
ax + b \qquad\overset{\text{subtract $b$}}{\rightarrow}\qquad
ax \qquad\overset{\text{divide by $a$}}{\rightarrow}\qquad
x
$$
 Thus, we start with the original equation, first subtract
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$\,b\,$ from both sides, and then divide both sides by
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$\,a\,$:
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$ax + b = c$

(original equation) 
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$ax = c  b$

(subtract $b$ from both sides) 
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$\displaystyle x = \frac{cb}{a}$

(divide both sides by $a$) 
Master the ideas from this section
by practicing the exercise at the bottom of this page.
When you're done practicing, move on to:
Getting Bigger? Getting Smaller?