(x+3)² = (x+3)(x+3)
When we distribute polynomials, we have to make sure each term mutliplies to all of the terms in the other polynomial.
x(x+3) + 3(x+3)
x² + 3x + 3x + 9
x² + 6x + 9
Methods such as FOIL (first, outside, inside, last) and creating a table can help you with this.