Physics, how to do(a), (b)?

(a) In the 1st 5 sec, the car starts from 0 ms⁻¹, accelerating at 25/5 = 5 ms⁻², to 25 ms⁻¹
It moves on at this constant velocity of 25 ms⁻¹ for 10 sec,
Then, it decelerates at -25/(25-15) = -2.5 ms⁻², until it stops after another 10 sec.
In the next 5 sec, it travels backwards*, accelerating at 20/5 = 4 ms⁻², to 20 ms⁻¹
It moves on at this constant velocity of 20 ms⁻¹ for 5 sec,
Finally, it decelerates at -20/(45-35) = -2 ms⁻², until it stops after 10 sec.

(b) Total distance travelled
= total area under the graph
= ½ [(15-5)+25]x25 + ½ [(45-25)+5]x20
= 437.5 + 50
= 687.5 m

* Explanation:-
==========
The –ve sign of “velocity” indicates an OPPOSITE DIRECTION of the movement of the car, so that it is travelling backwards !