\(\displaystyle{\lim_{x \to \infty}}\dfrac{x^2+4x+3}{x+1}-(\alpha x + \beta)=3\)
\(\displaystyle{\lim_{x\to \infty}}\dfrac{(x+3)(x+1)}{x+1}-(\alpha x+\beta)\)
\(\displaystyle{\lim_{x\to \infty}}(x+3)-(\alpha x+\beta)=3\)
\(\displaystyle{\lim_{x\to \infty}}(1-\alpha)x+(3-\beta)=3\\ \\ \alpha = 1 \\ \beta = 0\)
.