how to find equation of polynomial from a graph
That depends. Do you have the graph to show us?
A polynomial of degree n can always be expressed as
$\displaystyle f(x) = a(x - r_1)(x - r_2)...(x - r_n)$
where the r's are the x-intecepts of the graph. The problem is that roots are not always real, and thus do not show up on the graph.
It would be best if you showed us your problem so we could tell you what cases you need to consider, etc.
-Dan