Graph of pdf and calculate moments 
http://www.dumpt.com/img/viewer.php?file=0pt0l719ug05g1r09sly.jpg
The diagram in the link shows the graph of a probability density function. Given that the mean is zero, express $b$ in terms of $a$. Calculate the second and third moments in terms $a$. 

I have found that $b=2a$. I am not sure how to find moments from a pdf graph. 

I am pretty sure your value for $b$ is wrong. How did you get it? 
There's enough information to find the pdf. From there you can find moments.
