The equations could be
(2n+7)+(2n+7)+3n
or 2(2n+7)+3n
When we combined like terms it could be
4n+14+3n
2(2n+7)+2(3n) is incorrect because in a triangle there are 3 sides, the measure of two of the sides are 2(2n+7) and the third side is 3n however, he/she put "2(3n)" which would show there are 2 sides with the measure 3n