How do i determine an objects velocity by looking at a acceleration vs time graph?

how do i determine an objects velocity at certain times by looking at a acceleration vs time graph
Update: http://session.masteringphysics.com/problemAsset/1070272/3/02.P28.jpg
heres the graph and i need to know wat the velocity at times 2s,4s,6s,and 8s .... i cant seem to come up with the right answers
Update 2: im getting 0,5,20,-30,0 and its telling me its wrong
3 answers 3