The slope formula is the difference of the y values over the difference in the x values. We have a slope, so we will fill in the formula with what we have and then solve for x: 
. Multiply both sides by x-10 to get rid of the fraction. -2(x-10) = 4. Distribute to get -2x+20=4 and -2x = -16. That gives us an x value of 8.