Field of a point charge

Two point charges, +Q and −Q, are fixed 0.40 m apart in vacuum. Point P lies on the line joining them, 0.10 m from the +Q charge and 0.30 m from the −Q charge. The electric field strength at P is $1.5\times10^{5}$ V m⁻¹ directed towards the −Q charge. Using $E = \dfrac{1}{4\pi\varepsilon_{0}}\dfrac{Q}{r^{2}}$, calculate the magnitude of Q.