[tex]d=2r\\
A=128 \pi \\
A=2 \pi r^{2} +2 \pi rh \\\\
2 \pi r^{2} +2 \pi rh = 128 \pi \\
2 r^{2} +2rh = 128\\
2 r^{2}+24r-128=0\\
r^{2}+12r-64=0\\
delta=400\\
r_{1/2} = -16/4 \\
r=4cm\\
d=8cm\\
[/tex]
Radius / diameter as a length cannot be negative, that's why '-16' is wrong.
