To solve this problem you must apply the proccedure shown below:
1. You have the followin quadratic equation given in the problem above: x^2+3x+1
2. Therefore, you have:
x^3+3x=-1
(b/2)^2=(3/2)^2=9/4
x^2+3x+9/4=-1+9/4
(x+3/2)^2=5/4
Therefore, as you can see, the answer is: (x+3/2)^2=5/4