Three point charges q, 2q, Q are arranged along a line as shown in the figure. If q = 2.0 C and the electric field at point X is zero, what is Q?

Can someone explain how to solve this? I don't understand this explanation.

3. Three point charges q, 2q, Q are arranged along a line as shown in the 2q figure. If q = 2.0 uC and the electric field at point X is zero, what is Q? Answer: -8.0 JC Solution: Ex at point X from the charges q and 2q is -kq/d'. At point X the field from O must have the same magnitude and be in the opposite direction. O must therefore satisfy the rela- tion kQ/4d2 = -kq/d2 or Q = -4q = -8 /C