Marcus finds that open paren 3 x squared minus 2 y squared plus 5 x close paren plus open paren 4 x squared plus 12 y squared minus 7 x close paren is equal to 7 x squared minus 10 y squared minus 2 x What error did Marcus make?

When adding the polynomials, combine like terms: the y² terms are −2y² + 12y² = 10y². The correct sum is 7x² + 10y² − 2x, so Marcus combined −2y² and 12y² incorrectly. The correct choice is “He combined the terms negative 2 y squared and 12 y squared incorrectly.”

AI-written and checked against the verified answer.