Question 343293
When you have an equation, you can add or subtract, multiply and divide, by any number as long as you do it to both sides of the equation (except division by zero).
You want to isolate all the x terms on one side, all the constant terms on the other. 
Eventually you want an equation that looks like, x=...., because then you're done, you've solved the problem.
Here's an example.
{{{5x+13=2x-4}}}
Add {{{-2x}}} to both sides, to remove it from the right hand side.
{{{5x-2x+13=2x-2x-4}}}
{{{3x+13=0-4}}}
{{{3x+13=-4}}}
Subtract {{{13}}} from both sides, to remove it from the left hand side.
{{{3x+13-13=-4-13}}}
{{{3x+0=-17}}}
{{{3x=-17}}}
Divide both sides by {{{3}}}.
{{{(3x)/3=-17/3}}}
{{{(3/3)x=-17/3}}}
{{{(1)x=-17/3}}}
{{{highlight(x=-17/3)}}}
You're done, you've solve for x. 
You can verify the solution by plugging it into your original equation.
{{{5x+13=2x-4}}}
{{{5(-17/3)+13=2(-17/3)-4}}}
{{{-85/3+13=-34/3-4}}}
Multiply both sides by 3 to eliminate the fractions.
{{{-85+39=-34-12}}}
{{{-46=-46}}}
True, so the solution is valid.
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Not all equations have a solution.
{{{5x=5x+3}}}
Subtract {{{5x}}} from both sides.
{{{5x-5x=5x-5x+3}}}
{{{0=0+3}}}
{{{0=3}}}
That's obviously not true, so no x solves that problem.
There is no solution.
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Here are your two examples without all of the steps.
Make sure you understand how to go from 1 step to the next.
{{{4x+15 = 30}}}
{{{4x+15-15=30-15}}}
{{{4x=15}}}
{{{x=15/4}}}
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{{{4x-6 = 25x}}}
{{{4x-25x-6=25x-25x}}}
{{{-21x-6=0}}}
{{{-21x=6}}}
{{{x=-6/21}}}
{{{x=-2/7}}}
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Good luck.