Marcus finds that (3x^2-2y^2+5x)+(4x^2+12y^2-7x)=7x^2 -10y-2x. what error did marcus make?

Amountain climber starts a climb at an elevation of 453 feet above sea level at his first rest stop he has climbed 162 feet and by his second rest stop he has climbed another 207 feet its getting late in the day so the climber starts his way down if the climber desends 285 feet how much does he need to ascend or descend to return to the original starting point