The x-intercepts are the points where the curve hits the x-axis. Do you mean you have trouble solving for ?
Think about using the derivative. When the derivative is in a point, the function is very likely to turn round at this point (although sometimes it doesn't).
- get the derivative of your function
- solve for with
- ensure each point found is indeed a turning point (you should know how to do this last step)
Does it make sense ?