Computer depot sold 762 reams of paper. staples sold 3 times as much paper as computer depot and 143 reams more than office supply central. how many reams of paper were sold by all 3 stores combined

What you want to do is multiply 762 by 3, do the order of operations, and then add 143.
The order of Operations is pretty simple.  Have you heard of GEMDAS?  Or even PEMDAS?  Well simply it is the same thing, however GEMDAS  is just involving grouping symbols to make it Grouping symbols, Exponents, Multiplication or Division (which ever one comes first, do it from left to right), and then Addition or Subtraction (again which ever one comes first, do it from left to right).  PEMDAS is the same thing, except it is Parenthesis, Exponents, Multiplication or Division (which ever one comes first, do it from left to right), and then last but not least Addition or Subtraction (again which ever one comes first, do it from left to right).  Hope that helped!  :)