There are two options for finding the zeros of a function in GeoGebra: The first is to use                                                                                                                                                                                                                                          CAS, and the other is to use the Algebra View and Graphics View. 
     GeoGebra Instruction 1
CAS CAS     under         View     in         Menu.         <     <Your     Expression>     =0>     in         CAS.         Solve Solve     Numerically 
      
     GeoGebra Instruction 2
Algebra View and Graphics View Algebra     View     and         Graphics     View     under         View     in         Menu.         Algebra     View     and         press         Enter.         Root(<Polynomial>)
and press Enter. 
Note! If your problem has a domain for the function, use Roots(<Function>, <Start x-value>, <End x-value>) instead. Type in the start and end values of the interval in the <Start x-value> and <End x-value> fields.