I would call this method incorrect.
If you're trying to solve for b, what you should do is first isolate b ² :
5 + b ² = 30
b ² = 30 - 5
b ² = 25
Then take the square root of both sides, noting that √(x ²) = |x| for all x :
√(b ²) = √25
|b| = 5
Then b = +5 or b = -5.
