Question 269712
You can get all wrapped up in the math on this one. But given the info provided, trial and error is a good way to go.
Try setting a=1 and seeing if it works
Try a=2
then a=3
Basically, get in the ball park and take it from there

Look at this for other variants and a plot
http://www.wolframalpha.com/input/?i=1%2Ba%2Ba^2%2Ba^3%3D40