You had $26 which should be your total to figure out how many pen you bought
The pens were $3.50 each so to find out how many we bought we have to put it next to a variable being 'x'
we have 8.50 left so that has to be added to our equation
now that we have our equation , we use order of operations to figure out x
3.50x +8.50=26 1. First we subtract 8.50 from our total
3.50x=17.50 2. we divide 3.50 from 17.50
x= 5
so you should have purchased 5 pens