if I'm in a plane flying at 512 miles per hour and a plane flies below me in the opposite direction, will it appear to fly slow or fast

It appears to fly faster than its actual speed.

Step-by-step explanation:

In general, if talking about velocities, the direction of the movement should also be taken into account. For example, if two objects move in opposite directions, a person inside one object observes that the other one moving in the opposite goes faster than its actual speed (because the velocities are summed up). If they are moving in the same direction the opposite phenomenon is true (the velocities are subtracted).

Finally, in this example the planes move in opposite direction, therefore, a plane flying in the opposite direction will appear to fly faster.