Question

how to find cubic equation if roots are given?

Answer 1

use the formula
x^3 + x^2(sum of zeroes) - x(sum of product of zeroes taken two at a time) + product of zeroes

Answer 2

using the trial and error method....

or 

use the formula
x^3 + x^2(sum of zeroes) - x(sum of product of zeroes taken two at a time) + product of zeroes