\(x^2+3x+x+3=\left(x+1\right)\left(x+3\right)\)
\(=x^2+x+3x+3=x\left(x+1\right)+3\left(x+1\right)=\left(x+1\right)\left(x+3\right)\)
\(x^2+x+3x+3=x\left(x+1\right)+3\left(x+1\right)=\left(x+1\right)\left(x+3\right)\)
x2+4x+3
= x2+x+3x+3
= (x2+x)+(3x+3)
= x(x+1)+3(x+1)
= (x+1)(x+3)