Question

What is this method called?

Answer 1

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.