Answer:

Work Shown:

Step-by-step explanation:
In the 2nd step, I multiplied both sides by k²
Then on the 4th step, I divided both sides by (a+b) to isolate k².
To fully isolate k itself, we apply the square root to both sides. The plus or minus is there because of an example like (-3)² = 9 and (3)² = 9.