The student only got half credit because he/she "did not write an explanation".
But there is an explanation. It is written in the language of mathematics!
The student clearly answers "yes" gives the reason saying, in essence. because that is what the solutions are, and then gives sound mathematical support for that claim.
The only flaw I might see is that "x" has drifted above the fraction line in one step, but in the pressure of an exam with limited time to carefully proofread and edit, I would excuse that.
In truth, this is a very very good response.
Side note: Reading Comprehension Passage B on the Regents Exam in English Language Arts (Common Core) is a translation of a poem originally in Chinese. I have nothing against translations, and I am currently reading Crime and Punishment for the third time with different translations.
But: translations on an English Language exam? Aren't there enough sources originally written in English? (Douglas Hofstadter wrote an excellent book on the issue of translations [Le Ton beau de Marot]. I highly recommend it.)