.

By the remainder and factor theorems, if is a root then .

So you should find:

.

After you have simplified these, you have 2 equations in 2 unknowns. Solve them simultaneously for and .

Once you have and , you have the polynomial.

Since is a root, is a factor.

Since is a root, is a factor.

Long divide your polynomials by these factors to find the third factor. From there you can find the final root .

If you have any more trouble, please show your working and exactly where you are stuck.