Describe the magnitude and direction of the vector or vectors in the following situation:

an airplane flying northwest at 175 knots

The difference between a scalar and a vector is that one is just a number, while the other represents a number pointed in some direction.

Speed is a scalar. It's just a magnitude and doesn't specify any direction.

Velocity is a vector. Its magnitude is the speed and its direction is northwest.

You could also look up the standard rules for compass directions and express the heading (direction) as an angle. North is defined as 0 degrees. The angle of a heading is the amount of clockwise rotation from north. Northwest is 45 degrees counterclockwise, so the direction would be -45 degrees (or +315 degrees).