I am slightly confused with your question. Are you looking for the equation or for x?
Anyway, here is both:
If you are finding the equation of the line, plug in your numbers in the equation y=mx+b. Use the point (2,4).
4=-2(2)+b
Solve for b.
4=-4+b
8=b
Then, put the slope and y-intercept in slope-intercept form.
y=mx+b.
y=-2x+8
Now you have the equation.
If you are solving for x, plug the point (x,8) in your equation.
y=-2x+8
8=-2x+8
Solve for x.
-2x=0
x=0
