You will already have learned about deceleration. We can also find this from speed-time graphs:
The red line shows a deceleration from 30 to 0 m/s in 10 s. The green line shows a slower deceleration from 10 to 0 m/s over 20 s.
As with accelerations, the gradient (or steepness) of the line indicates the deceleration.
For the red line this is found as follows:
For the green line the gradient is -10 / 20 = -0.5 m/s/s.
Note that both gradients are negative (they slope downwards) so these are decelerations, not accelerations.