An object moves with constant acceleration a = 2.5 m/s2 in the due North direction. At time t = 2, the velocity of the object is v0 = 5 m/s in the due East direction. What is the magnitude of the velocity at t = 4 s?

the way to get the answer is to use Pythagorean theorem, right? I am not sure about that.

Yes - the velocity in the east direction is constant 5 m/s, and the velocity in the north direction at t = 4s is 2.5m/s^2 x 4s = 10m/s. Use Pythagorus to find the resultant vector.