Q.Form the monic polynomial equation of degree $3$ whose roots are $2, 3, 6$.

To build the monic polynomial equation whose roots are a prescribed list of numbers $\alpha_1,\alpha_2,\ldots,\alpha_n$, form the product

and expand it. Equivalently, compute the elementary symmetric functions $S_1,S_2,\ldots,S_n$ of the given numbers and write

where the signs alternate, starting with $S_1$ negative. When one of the prescribed roots is irrational (such as $a+\sqrt b$) or non-real, the conjugate-pair theorems force its conjugate to be a root too if the final coefficients are to come out rational -- so an irrational or complex root is never handed to us alone; it always arrives together with its partner, and the pair contributes a single rational quadratic factor $\big(x-(a+\sqrt b)\big)\big(x-(a-\sqrt b)\big)=x^2-2ax+(a^2-b)$ to the product.