Strategy in solving quadratic equations by taking square roots

Problem

In order to finish building his catapult, Cyrus must solve the following equation.
space, start fraction, 1, divided by, 3, end fraction, left parenthesis, x, minus, 1, right parenthesis, start superscript, 2, end superscript, equals, 12
He completes the problem as seen in the steps below.

The catapult won't work! His friend tells him that the answer is x, equals, 7 and x, equals, minus, 5.
In which step did Cyrus make an error?
Please choose from one of the following options.