For this problem, you should use Pythagorean Theorem.
The formula for this is a^2 + b^2 = c^2.
The plane is moving north, and then moves to the right. Therefore, we need to find the hypotenuse, or resultant velocity. Plugging it into the equation we get, (245 m/s)^2 + (55 m/s)^2 = c^2
Solving for this, you will get 250 m/s.
