\displaystyle \lim_{n \to +\infty} \frac{3 n + 7}{4 n + 2} : \frac{n}{n} = \lim_{n \to +\infty} \frac{3 + \frac{7}{n}}{4 + \frac{2}{n}} = \frac{3 + 0}{4 + 0} = \frac{3}{4}