secondary 3 chevron_right E Maths chevron_right Singapore

As you can see I have shown you two different answers The one on top as my teacher’s answer and the one below is the one I tried to solve and I don’t understand what the answers are different is my teachers answer (the TOP one)wrong?I don’t understand why the second line the whole equation changes 15-7y-2y^2 suddenly becomes 2y^2+7y-15 I totally don’t understand the change in the second line for the plus and minus signs and my last answer you can see is y=5 while my teachers one is y=-5 someone please help and explain to me very clearly step-by-step thank you very very much!!