Consider f(x) = 1-e^x. find the slope of the graph of f(x) at the point where the graph crosses the x-axis.

       f ( x ) = 1 - e^x
      We have too find the slope of the graph at the point: f ` ( x o ), where x o is a coordinate of the point where the graph crosses the x-axis ). Also an equation of the tangent line is: y - y o = f ` ( x o ) ( x - x o ) , but we don`t need it here ( we just need to find the slope ).
       1 - e ^x = 0
        e ^x = 1     => x o = 0
       f ` ( x ) = - e ^x
       f` ( 0 ) = - e^0 = - 1 
       The slope is : m = - 1