y=x²-10x+21
y=x²-10x+25+21
y+25=x²-10+25+21
y+25=(x-5)²+21
y=(x-5)²+21-25
y=(x-5)²-4
0=(x-5)²-4
-(x-5)²=-4
(x-5)²=4
x-5=±2
x-5=-2
x-5=2
x=3
x=7
y=x²-10x+21
y=x²-10x+25+21
y=x²-10x+25+21
y+25=x²-10+25+21
y+25=(x-5)²+21
y=(x-5)²+21-25
y=(x-5)²-4
0=(x-5)²-4
-(x-5)²=-4
(x-5)²=4
x-5=±2
x-5=-2
x-5=2
x=3
x=7