Question 24158
To write an equation seems hard and most of the time we try to make it harder than it actually is.  So try not to stress over the problem, ok...
first remember that.  
you know her base salary and you know the amount she gets for selling barong sets so what you don't know is how many she will sell.  let this be your x or y or what ever variable you want.
she will get the 1500 no matter if she sells anything else or not so set up your equation like this

1500 + 35x= weekly salary.
even if she sells zero this formula will still work.