Alice is a white dog. She is two. Bob is a brown dog. Bob and Alice are good friends. Look at the two dogs.

Bob: Good morning!

Alice: Good morning!

Bob: What's this?

Alice: It's a jacket. It's my sister's. Oh, my sister is Grace.

Bob: What's that?

Alice: It's a book. It's Miss Miller's book. She is an English teacher.

Bob: Well, dogs don't like books.

  一元二次方程$x^2+2x-3=0$的二根$x_{1,}{x}_2$（$\left(x_1<x_2\right)$）
是抛物线$y=a{x}^2+b{y}^2+c$与$x$轴的两个交点B,C的横坐标，
且此抛物线过点A(3,6)．
（1）求此二次函数的解析式．
（2）用配方法求此抛物线的顶点为P .对称轴
（3）当x取什么值时， y随x增大而减小？

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