Forth and Back

Today I got to college just on time. Parked my car in A block, and run to C Block. But no one there. I rushed back to A block, last Wednesday we were there for the lecture. But no. After going back and forth I stopped to think where could they be. At the end, I got it and I got there... in B Block. It dawned on me that for now, every Tuesday we should be in the Mac Room to use SketchUp.
I really don't remember this information, probably it was given after I left, as I usually leave earlier.
I logged on my Mac - Then: FRUSTRATED!
1) Apple/Shift/3 doesn't work for me to create a screen shot!
2) The measurements don't add up!
I needed to start everything from scratch!!!!
I did everything  again and now it's almost fine.... but something must be wrong with the measurements given on the plan... I'll check it out with the tutor.
*I am generally good with the computer and I have perhaps underestimate SketchUp.