Answer
$(a)\space 7.79\times10^{14}W/m^{2}$
$(b)\space 3.12\times10^{9}W/m^{2}$
$(c)\space 904.08\space W/m^{2}$
Work Step by Step
Here we use the equation $I=\frac{P}{4\pi r^{2}}$ where $I$ - Wave intensity, r - radius of the sphere
(a) $I=\frac{P}{4\pi r^{2}}$ ; Let's apply $P = 120 GW,\space r =3.5mm$
$I=\frac{120\times10^{9}W}{4\pi\times(3.5\times10^{-3}m)^{2}}=7.79\times10^{14}W/m^{2}$
(b) $I=\frac{P}{4\pi r^{2}}$ ; Let's apply $P = 120 GW,\space r =(3.5m\div2)=1.75m$
$I=\frac{120\times10^{9}W}{4\pi\times(1.75m)^{2}}=3.12\times10^{9}W/m^{2}$
(b) $I=\frac{P}{4\pi r^{2}}$ ; Let's apply $P = 120 GW,\space r =(6500m\div2)=3250m$
$I=\frac{120\times10^{9}W}{4\pi\times(3250m)^{2}}=904.08\space W/m^{2}$
