We know that Resistance Formula :
R = resistance 
Ω 
ρ = specific resistance 
Ω×m 
l = length of the cable 
m 
A = cross section area 
mm^{2}

We know that resistance is inversely proportional to square of radius of the wire.
So if the radius of cross section is decreased by x then resistance will be increased by x^{2.}
So in the given question initial radius is 9mm and the radius is decreased by 3mm.
Hence the resistance will be increased by 9 times the initial resistance of the single cable.
So the resistance of the new cable will be R= 9*10= 90 ohm