You had 26$ in your pocket. you purchased x pens at $3.50 each. you now have $8.50 in your pocket. write and solve an equation to find the number of pens purchased

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