(c) Will either of you be speeding? (speed limit 50 km/hr)
You're traveling at 40 km/hr, which is under the speed limit.
To figure out how fast the Porsche is going at t = 5.631 seconds, use:
v = vo + a t = 0 + (3) (5.631) = 16.893 m/s.
Converting to km/hr gives 60.8 km/hr, so the Porsche is speeding.