We can find for the new speed of the plane using the cosine law. Let us say that the new speed is c:
c^2 = 280^2 + 30^2 – 2(280)(30) cos (90°+45°)
c = 302 mi/hr
The angle relative to the ground is calculated using sine law:
sin C / 30 = sin (90+45) / 302
C = 4.03°
The new speed is 302 mi/hr at -4.03° relative to the ground.