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...

Let x = the number of White Sweatshirts,
Let y = the number of Yellow Sweatshirts

Now that we know that x and y are the NUMBER OF SWEATSHIRTS SOLD, how many were sold total? 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...

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.

The same is true for yellow sweatshirts, 12 dollars times y number of yellow is 12x.

When you add the two profits together you should get the total profit, so

Here are you two equations then.

Now use any method you are comfortable with to solve this problem, Substitution, Elimination or the Calculator.

If you are comfortable using matrices and you have a TI 83 or 84 calculator try this method. Under matrix create a 2x3 matrix.

in the first row plug in 1 1 30
in the 2nd row plus in 10 12 344

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.

Plug in the matrix you just created...

rref([A])

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.

Hope that helps, Mr. C