Well I taught maths for 15 years and you've managed to confuse me!
Firstly, b and c don't seem to have anything to do with your question (since you state their values but then never mention them again!).
Then you state something which totally contradicts the original information. 2a means '2 times a' and, since a =2, 2a must be equal to 4, not 10.
Typing out maths notation, here on AB, can be very difficult. If you've got a scanner, please scan the actual page with the question on, and save your image as a jpg file. Then upload the picture to here:
http://www.tinypic.com/
(It's incredibly easy to do that. Just click 'Browse'. Navigate to the file's location on your hard drive. Double click on the file. Click 'Upload Now')
Then highlight the 'direct link' (by dragging your cursor over it while holding down the left button). Right-click on it and select 'copy'. Come to this thread and click 'Post an answer'. Right-click in the text area and select 'paste'. Click 'Submit'.
When you've done that, we'll be able to see exactly what the question says, so that we can provide an answer.
Chris