The answer is the first choice: 7, 15, 20.
When you want to know what three numbers can make up a triangle. All you have to do is take the first two numbers and add them up. If the sum is larger than the third number, it can form a triangle. If the sum of the first two numbers are smaller than the third number, it cannot form a triangle.
Hope this helped
Hey There!
The answer to your problem is
or
both answers are correct!
<u>Step 1: Subtract x²+12 from both sides.</u>
−7x−(x²+12)=x²+12−(x²+12)
−x²−7x−12=0
<u>Step 2: Factor left side of equation.</u>

<u>Step 3: Set factors equal to 0.</u>
or 
<u>Get your answer:</u>
or 