设对称点的坐标为P(x,y)点(2,5)与点P(x,y)中点坐标为[(x+2)/2,(5+y)/2]
(y-5)/(x-2)=-3
(x+2)/2-3*(5+y)/2+3=0
解得x=4,y=-1
所以P(4,-1)