"A number" is unknown so just create a variable in this case I made it "n."
We know that sum means to add so add the variable n and 7 then going back up the beginning we see it says twice so we take that sum and times is by two and I'm using parentheses to show this. The next part of the problem says "Is" and that's an indication of an equal sign. We'll then put the equal sign after the problem we just filled out and finish off the equation with 3 times the number meaning we'll put the 3 and next to each other as another way of multiplying.
