Solve for c. c^2−9=b^2 (choices in photo)

c = ± 

Explanation:
Before we begin, remember the following:
(x)² = (-x)² = x²
√x² = ±x

Now, for the given:
To solve for c means that we need to isolate the c on side of the equation.
This can be done as follows:
c² - 9 = b² 
Add 9 to both sides of the equation:
c² - 9 + 9 = b² + 9
c² = b² + 9
Now, take the square root for both sides of the equation. Don't forget the the square root gives a positive and a negative value.
√c² = √(b²+9)
c = ± 

Hope this helps :)