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The main point is that only the vertical distance matters when determining pressure changes in manometers. Inclined tube manometers usually have distance markings along the length of the tube, so whomever designs the tubes will have to account for this.
pressure only changes due to change in elevation. "h_2" is given which is the elevation change from C to D. Now, from point D to E we do not know the elevation change but we do have the length the fluid travels, "L" and the angle "theta" and using trigonometry we can find the vertical distance from point D to E.
