Parker was given this problem on his homework: 6x2−4y+2x+4x2+8y. Here is the work he showed to simplify the expression by combining like terms.

6x2−4y+2x+4x2+8y
6x2+4x2=10x2
10x2+2x−4y+8y
4y+8y=12y
10x2+12y+2x
Parker's final answer was 10x2+12y+2x, but the correct answer should have been 10x2+4y+2x.

In at least 5 complete sentences, tell me where Parker made his mistake and how he can fix it to get the correct answer.