As a salesperson at scrapbooks and treasures, alysse receives a monthly base pay plus commision on all that she sells. if she sells $400 worth of merchandise in one month, she is paid $388. if she sells $1,000, she is paid $520. find alysse's salary when she sells $3,300 worth of merchandise.

In order to find the answer to this, you need to be able to solve for 2 things: what she makes as a base salary and what she makes percentage-wise on her commission.  You have information for 2 different equations that will help you here.  Set them up as equations, using x and her base salary and y as the percentage of her sales:
where 388 is what she earns after her base salary, x, is paid and her commission of 400.  y is the percent of the commission that she earns.  As you can see, we have 2 unknowns, which COULD be a problem.  But let's move on first before we freak out.
The next equation involves the other bit of info: that she earns 520 after her base salary of x and her commission of 1000 based on whatever the percentage of her sales is, which is y.  That equation looks like this:

Because her base salary is constant, the x's are the same number in both equations.  So we can solve each for x:


Now that these are both solved for x, and x is the exact same number, we can set those 2 equations equal to each other by the transitive property of equality:
=388+400y= -520+1000y
Solving for y gives you:
132=600y and

Because this is a percentage, we will multiply the decimal we get by 100 to find that the percentage she earns on a sale is 22%.  Now that we have that percentage, we can find her base salary.  Do that like this:
Use one of your equations to solve for x:
388=x+[(.22)(400)]
388=x+88 and x = 300.  That's her base salary.  Now use that base salary in an equation to find out her monthly pay after she sells $3,300 worth of merchandise in a month:
x = 300 + [(.22)(3300)]
x = 300 + 726
x = $1,026
That's how much she makes after her base salary of $300 plus her earning off selling $3,300 worth of merchandise at 22% commission.