Remember that the integral represents area under the curve (or between the curve and the x-axis, thus area above the curve counts as negative area.
Also, areas add to each other so
1∫ 3f(x) dx =1 ∫2f(x) dx + 2∫3f(x) dx
and -2a∫bf(x) is just -2 times the area for that region.
Let me know if this helps.