\(\displaystyle \boxed{(a+b)=a^2+2ab+b^2} \\ \\ (x+3)^2+2(x+3)= \\ x^2+2 \cdot x \cdot 3 +3^2+2x+6 = \\ x^2+6x+9+2x+6= \\ x^2+8x+15= \\ \\ \Delta= b^2-4ac \\ \Delta=8^2-4 \cdot1 \cdot15 \\ \Delta=64-60=4 \\ \\ \\ X_{1,2}= \frac{-b\pm \sqrt{\Delta} }{2a} = \frac{-8\pm2}{2} = \\ \\X1=5 \\ X2=3\)
