Question 131481
Hopefully you know that you are solving a system of equations here and that you must set up 2 equations with 2 variables.
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The first think you need to do is determine what your two variables are, in this case you are selling White Sweatshirts and Yellow Sweatshirts, so...
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Let x = the number of White Sweatshirts, 
Let y = the number of Yellow Sweatshirts
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Now that we know that x and y are the NUMBER OF SWEATSHIRTS SOLD, how many were sold total? 30

{{{ x + y = 30 }}}

The other information we have is about the cost, $10 for a White and $12 for a yellow. This next equation is exactly the same way a Cash Register figures out the problem...
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10 dollars for every While sweatshirt sold, if you sold 3 you would have 3*10 which is 30 dollars. But since we don't know how many sweatshirts were sold we have to use our variable, so 10 dollars times x number of sweatshirts is 10x.
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The same is true for yellow sweatshirts, 12 dollars times y number of yellow is 12x.
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When you add the two profits together you should get the total profit, so
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{{{ 10x + 12y = 344 }}}

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Here are you two equations then.
{{{ x + y = 30 }}}
{{{ 10x + 12y = 344 }}}

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Now use any method you are comfortable with to solve this problem, Substitution, Elimination or the Calculator.
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If you are comfortable using matrices and you have a TI 83 or 84 calculator try this method. Under matrix create a 2x3 matrix. 
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in the first row plug in  1   1   30
in the 2nd row plus in   10  12  344
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Quit out and go back to your home screen, Hit the 2nd and then Zero to get to the catalog, hit the times button ( x ) to get to the r's and scroll down to find the rref( function.
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Plug in the matrix you just created...
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rref([A])
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Hit enter, and you should have the answer ...

1               0               8
0               1               22

There ya go, an answer to your question AND a fun exciting way to use the calculator.
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Hope that helps, Mr. C