We are asked to write a certain equation. Your first task is to read the statement thoroughly. If you've notice, the statement mentioned "a number". Let's assign x to be the number. First statment is twice a number (2x). Then, decreased by the quotient of that number and 2 (-x/2). Lastly, the statement at least 12 is written as >12.
2x - (x/2) >12
