Your approach was kind of correct.

View attachment 32753
By solving for

you would have gotten the area that is shaded in black.

Rather than doing that the answers give a more interesting method.

View attachment 32754
The area in blue represents

since the graph is symmetrical we can also take the purple area to be the same. Looking at both the green and the purple areas we can see that they form a right angle triangle which has the area of

. Then to find the green area we just simply subtract the purple area from the area of the triangle