It helps a lot to draw a picture. Although the problem does not specify how the points are arranged, I assume they are in a line. Your picture should be a line with points a, b, and c on the line. Now write 3x above the ab segment and 5x above the bc segment. Draw one of those fancy brackets { encompassing the entire line and write 8 on the tip. Now that you have your picture, you can see that ab + bc = ac, which means 3x + 5x = 8. Does that help?
