Answer:
1 and -3.
Step-by-step explanation:
Given the quadratic polynomial:

To use the completing the square method to find the zeros, follow the steps below:
Step 1: Take the constant to the right-hand side.

Step 2: Divide the coefficient of a by 2, square it and add it to both sides.

Step 3: Write the left-hand side as a perfect square.

Step 4: Take the square root of both sides.
![a+1=\pm\sqrt[]{4}](https://img.qammunity.org/2023/formulas/mathematics/college/xmcf7zk5r8d5cb81xi5zu8jb2rk7ghvmea.png)
Step 5: Solve for a.
![\begin{gathered} a=-1\pm\sqrt[]{4} \\ a=-1\pm2 \\ a=-1+2\text{ or }a=-1-2 \\ a=1\text{ or }a=-3 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/abjvypyey446l22hbcuyr7bpx2fi42sp7d.png)
The zeros of the quadratic equation are 1 and -3.