Answer: -2
Explanation: You're able to notice by looking at the y coordinates of both "c" and "d" that they are six units apart. This, therefore, means that we're able to draw a vertical line down from "c" that's six units long. Now you're able to notice we can form a triangle by drawing a line from "d" to the line that we just drew. This line would be eight units. How do we know this? Well, you're able to use the pythagorean theorem (a^2 + b^2 = c^2) to solve for it. Keep in mind this:
a and b = triangle leg
c = longest line also known as the hypotenuse (the one that's marked as 10)
Since we know c, which is 10, and a, which is 6, we can now substitute it in.
6^2 + b^2 = 10^2
Solve for it and you would get eight.
This now means that point "c" is eight units away from point "d", so we have to add eight to the -10, which will hand us -2 as the x coordinate for "d".
Hopefully this helped at least a bit. Grammar may be a little messed up as I was about to head off to bed. Have a nice day!