5x^{2} - 2x + 10 = (x-10)^2

5x²-2x+10=(x-10)²
5x²-2x+10=x²-20x+100
5x²-x²-2x+20x+10-100=0
4x²+18x-90=0
2x²+9x-45=0
D=9²-4.2.(-45)=81+360=441
√D=√441=21
x1=(-9+21)/4=12/4=3, x1=3
x2=(-9-21)/4=-30/4=-7,5, x2=-7,5