This is an exercise in computation and there is little if any real math involved.
The problem has the solution backwards: first graph the equation to get the approximate roots of which there are 3. Then use the approximate roots to do Newton's Method/
The critical piece of Newton's method is f'(x)=-3x2 + 5.4x + 3.55
Remember Newton's Method says: xn+1 = xn + f(xn)/f'(xn)