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Squaring both sides removes the radical, giving ax + b = c² and hence x = (c² − b)/a. Squaring is not reversible, so the answer must always be substituted back to rule out extraneous roots.
Radical equation
√(ax + b) = c ⇒ ax + b = c² ⇒ x = (c² − b)/a, valid only if c ≥ 0
2x + 3 = 25, so x = 11, and √25 = 5 confirms it.
One produced by squaring that fails in the original equation, typically because it makes a square root equal a negative number.