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 x 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)