GRE Sample Test 2 Quantitative Reasoning 1

Always draw a figure when one is not provided. In this case, line segment XZ has a length of 68. Point Y is the midpoint of the segment, because 2XY = XZ. To find the lengths of these segments, divide 68 by 2.
Segment YZ = 34. Because YZ = 4a + 6, you know that 34 = 4a + 6, so a = 7.