The easiest way to answer this is to plug in all of the given choices to see which of them will satisfy the given equation.
We get,
Choice A: 7
Therefore, x is not equal to 7.
Choice B: 3
Therefore, x is not equal to 3.
Choice C: -2
Therefore, x is not equal to -2.
Choice D: -7
Therefore, x = -7
The answer is letter D : -7