f(x)= -4(x-1)^2+25
f(x)=-4(x^2-2x+1)+25
f(x)=-4x^2+8x-4+25
f(x)=-4x^2+8x+21
a=-4, b=8, c=21
\Delta=b^2-4ac=64-4*(-4)*21=64+336=400
\sqrt\Delta=20
x_1=\frac{-b-\sqrt\Delta}{2a}=\frac{-8-20}{2*(-4)}=\frac{-28}{-8}=\frac{7}{2}=3,5
x_2=\frac{-b+\sqrt\Delta}{2a}=\frac{-8+20}{-8}=\frac{12}{-8}=-\frac{3}{2}=-1,5