Answer
The charge on the sphere is $~~-7.5\times 10^{-9}~C$
Work Step by Step
We can draw a Gaussian sphere with radius $r = 15~cm$ with the same common center as the sphere.
We can find the magnitude of the sphere's charge:
$\epsilon_0~\Phi = q_{enc}$
$(\epsilon_0)~(E)(4\pi~r^2) = q_{enc}$
$q_{enc} = (8.854\times 10^{-12}~F/m)~(3000~N/C)(4\pi)(0.15~m)^2$
$q_{enc} = 7.5\times 10^{-9}~C$
Since the electric field is radially inward, the charge on the sphere is $~~-7.5\times 10^{-9}~C$
