To solve this, you need to subtract. Words like "take away", "give away", and "minus" are all clues that you should subtract. The number of pens Bob started with, which is three) will be the first number in our subtraction problem. The second number will be the number he gave away (one).
If you're stuck on a word problem like this, imagine you're actually doing it. Take three pens and then remove one. You are taking away, so the problem is subtraction, and then you can count how many you have left. After that, you'll express what you did in numbers.
I had three pens (3) and I gave away one (-1), so I have two left (=2). So (3-1=2).