Your agent can provide additional information--you can file an assessment appeal on line- http://-www.nassaucountyny.gov/arc do keep in mind that by filing an appeal does not necessarily mean you'll get a reduction.
First thing is to make sure that you apply for the STAR program which typically should save you approximately $1100 on the school portion of your real estate taxes. The next thing to do is to challenge the assessed valuation of your home by applying on line at the Nassau County Tax Assessor's office. You are probably being overassessed at least $100,000 possibly more. If you are not comfortable challenging the assessment yourself you can always hire a property tax reduction service or attorney such as Fred Perry, who specializes in this. Good luck!