Here, coefficient of x² = 3, coefficient of x = 10 and constant term = 3

We shall now split up the coefficient of x i.e., 10 into two parts whose sum is 10 and product is 3 * 3 = 9

So, we write middle term 10x as 9x + x

Thus, we have

3x² + 10x + 3 = 3x² + 9x + x + 3

= 3x (x + 3) + 1 (x + 3)

= (3x + 1) (x + 3)