Wasn't for sure if this should be posted here or in the Linear Algebra forum. It's in my Linear Algebra class but is still fairly basic stuff I think..

I can't seem to get the correct answer for question 47 from this page and can't figure out what I am doing wrong. I tried using the formula given and also tried using the basic 1/2bh (which gives the same formula):

Here's my work (a bit all over the place but should be readable, any questions ask.)

My final answer would have been but simplified or whatever. The answer in the back is though and I can't seem to find my mistake.

Thanks in advance guys (or girls.)